fs/mpage.c - pub/scm/linux/kernel/git/martin.lau/bpf-next - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * fs/mpage.c
  *
  * Copyright (C) 2002, Linus Torvalds.
  *
  * Contains functions related to preparing and submitting BIOs which contain
  * multiple pagecache pages.
  *
  * 15May2002	Andrew Morton
  *		Initial version
  * 27Jun2002	axboe@suse.de
  *		use bio_add_page() to build bio's just the right size
  */

 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/kdev_t.h>
 #include <linux/gfp.h>
 #include <linux/bio.h>
 #include <linux/fs.h>
 #include <linux/buffer_head.h>
 #include <linux/blkdev.h>
 #include <linux/highmem.h>
 #include <linux/prefetch.h>
 #include <linux/mpage.h>
 #include <linux/mm_inline.h>
 #include <linux/writeback.h>
 #include <linux/backing-dev.h>
 #include <linux/pagevec.h>
 #include "internal.h"

 /*
  * I/O completion handler for multipage BIOs.
  *
  * The mpage code never puts partial pages into a BIO (except for end-of-file).
  * If a page does not map to a contiguous run of blocks then it simply falls
  * back to block_read_full_folio().
  *
  * Why is this?  If a page's completion depends on a number of different BIOs
  * which can complete in any order (or at the same time) then determining the
  * status of that page is hard.  See end_buffer_async_read() for the details.
  * There is no point in duplicating all that complexity.
  */
 static void mpage_end_io(struct bio *bio)
 {
 	struct bio_vec *bv;
 	struct bvec_iter_all iter_all;

 	bio_for_each_segment_all(bv, bio, iter_all) {
 		struct page *page = bv->bv_page;
 		page_endio(page, bio_op(bio),
 			   blk_status_to_errno(bio->bi_status));
 	}

 	bio_put(bio);
 }

 static struct bio *mpage_bio_submit(struct bio *bio)
 {
 	bio->bi_end_io = mpage_end_io;
 	guard_bio_eod(bio);
 	submit_bio(bio);
 	return NULL;
 }

 /*
  * support function for mpage_readahead.  The fs supplied get_block might
  * return an up to date buffer.  This is used to map that buffer into
  * the page, which allows read_folio to avoid triggering a duplicate call
  * to get_block.
  *
  * The idea is to avoid adding buffers to pages that don't already have
  * them.  So when the buffer is up to date and the page size == block size,
  * this marks the page up to date instead of adding new buffers.
  */
 static void
 map_buffer_to_page(struct page *page, struct buffer_head *bh, int page_block)
 {
 	struct inode *inode = page->mapping->host;
 	struct buffer_head *page_bh, *head;
 	int block = 0;

 	if (!page_has_buffers(page)) {
 		/*
 		 * don't make any buffers if there is only one buffer on
 		 * the page and the page just needs to be set up to date
 		 */
 		if (inode->i_blkbits == PAGE_SHIFT &&
 		    buffer_uptodate(bh)) {
 			SetPageUptodate(page);
 			return;
 		}
 		create_empty_buffers(page, i_blocksize(inode), 0);
 	}
 	head = page_buffers(page);
 	page_bh = head;
 	do {
 		if (block == page_block) {
 			page_bh->b_state = bh->b_state;
 			page_bh->b_bdev = bh->b_bdev;
 			page_bh->b_blocknr = bh->b_blocknr;
 			break;
 		}
 		page_bh = page_bh->b_this_page;
 		block++;
 	} while (page_bh != head);
 }

 struct mpage_readpage_args {
 	struct bio *bio;
 	struct page *page;
 	unsigned int nr_pages;
 	bool is_readahead;
 	sector_t last_block_in_bio;
 	struct buffer_head map_bh;
 	unsigned long first_logical_block;
 	get_block_t *get_block;
 };

 /*
  * This is the worker routine which does all the work of mapping the disk
  * blocks and constructs largest possible bios, submits them for IO if the
  * blocks are not contiguous on the disk.
  *
  * We pass a buffer_head back and forth and use its buffer_mapped() flag to
  * represent the validity of its disk mapping and to decide when to do the next
  * get_block() call.
  */
 static struct bio *do_mpage_readpage(struct mpage_readpage_args *args)
 {
 	struct page *page = args->page;
 	struct inode *inode = page->mapping->host;
 	const unsigned blkbits = inode->i_blkbits;
 	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
 	const unsigned blocksize = 1 << blkbits;
 	struct buffer_head *map_bh = &args->map_bh;
 	sector_t block_in_file;
 	sector_t last_block;
 	sector_t last_block_in_file;
 	sector_t blocks[MAX_BUF_PER_PAGE];
 	unsigned page_block;
 	unsigned first_hole = blocks_per_page;
 	struct block_device *bdev = NULL;
 	int length;
 	int fully_mapped = 1;
 	int op = REQ_OP_READ;
 	unsigned nblocks;
 	unsigned relative_block;
 	gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);

 	if (args->is_readahead) {
 		op |= REQ_RAHEAD;
 		gfp |= __GFP_NORETRY | __GFP_NOWARN;
 	}

 	if (page_has_buffers(page))
 		goto confused;

 	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
 	last_block = block_in_file + args->nr_pages * blocks_per_page;
 	last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
 	if (last_block > last_block_in_file)
 		last_block = last_block_in_file;
 	page_block = 0;

 	/*
 	 * Map blocks using the result from the previous get_blocks call first.
 	 */
 	nblocks = map_bh->b_size >> blkbits;
 	if (buffer_mapped(map_bh) &&
 			block_in_file > args->first_logical_block &&
 			block_in_file < (args->first_logical_block + nblocks)) {
 		unsigned map_offset = block_in_file - args->first_logical_block;
 		unsigned last = nblocks - map_offset;

 		for (relative_block = 0; ; relative_block++) {
 			if (relative_block == last) {
 				clear_buffer_mapped(map_bh);
 				break;
 			}
 			if (page_block == blocks_per_page)
 				break;
 			blocks[page_block] = map_bh->b_blocknr + map_offset +
 						relative_block;
 			page_block++;
 			block_in_file++;
 		}
 		bdev = map_bh->b_bdev;
 	}

 	/*
 	 * Then do more get_blocks calls until we are done with this page.
 	 */
 	map_bh->b_page = page;
 	while (page_block < blocks_per_page) {
 		map_bh->b_state = 0;
 		map_bh->b_size = 0;

 		if (block_in_file < last_block) {
 			map_bh->b_size = (last_block-block_in_file) << blkbits;
 			if (args->get_block(inode, block_in_file, map_bh, 0))
 				goto confused;
 			args->first_logical_block = block_in_file;
 		}

 		if (!buffer_mapped(map_bh)) {
 			fully_mapped = 0;
 			if (first_hole == blocks_per_page)
 				first_hole = page_block;
 			page_block++;
 			block_in_file++;
 			continue;
 		}

 		/* some filesystems will copy data into the page during
 		 * the get_block call, in which case we don't want to
 		 * read it again.  map_buffer_to_page copies the data
 		 * we just collected from get_block into the page's buffers
 		 * so readpage doesn't have to repeat the get_block call
 		 */
 		if (buffer_uptodate(map_bh)) {
 			map_buffer_to_page(page, map_bh, page_block);
 			goto confused;
 		}

 		if (first_hole != blocks_per_page)
 			goto confused;		/* hole -> non-hole */

 		/* Contiguous blocks? */
 		if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
 			goto confused;
 		nblocks = map_bh->b_size >> blkbits;
 		for (relative_block = 0; ; relative_block++) {
 			if (relative_block == nblocks) {
 				clear_buffer_mapped(map_bh);
 				break;
 			} else if (page_block == blocks_per_page)
 				break;
 			blocks[page_block] = map_bh->b_blocknr+relative_block;
 			page_block++;
 			block_in_file++;
 		}
 		bdev = map_bh->b_bdev;
 	}

 	if (first_hole != blocks_per_page) {
 		zero_user_segment(page, first_hole << blkbits, PAGE_SIZE);
 		if (first_hole == 0) {
 			SetPageUptodate(page);
 			unlock_page(page);
 			goto out;
 		}
 	} else if (fully_mapped) {
 		SetPageMappedToDisk(page);
 	}

 	/*
 	 * This page will go to BIO.  Do we need to send this BIO off first?
 	 */
 	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
 		args->bio = mpage_bio_submit(args->bio);

 alloc_new:
 	if (args->bio == NULL) {
 		if (first_hole == blocks_per_page) {
 			if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
 								page))
 				goto out;
 		}
 		args->bio = bio_alloc(bdev, bio_max_segs(args->nr_pages), op,
 				      gfp);
 		if (args->bio == NULL)
 			goto confused;
 		args->bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
 	}

 	length = first_hole << blkbits;
 	if (bio_add_page(args->bio, page, length, 0) < length) {
 		args->bio = mpage_bio_submit(args->bio);
 		goto alloc_new;
 	}

 	relative_block = block_in_file - args->first_logical_block;
 	nblocks = map_bh->b_size >> blkbits;
 	if ((buffer_boundary(map_bh) && relative_block == nblocks) ||
 	    (first_hole != blocks_per_page))
 		args->bio = mpage_bio_submit(args->bio);
 	else
 		args->last_block_in_bio = blocks[blocks_per_page - 1];
 out:
 	return args->bio;

 confused:
 	if (args->bio)
 		args->bio = mpage_bio_submit(args->bio);
 	if (!PageUptodate(page))
 		block_read_full_folio(page_folio(page), args->get_block);
 	else
 		unlock_page(page);
 	goto out;
 }

 /**
  * mpage_readahead - start reads against pages
  * @rac: Describes which pages to read.
  * @get_block: The filesystem's block mapper function.
  *
  * This function walks the pages and the blocks within each page, building and
  * emitting large BIOs.
  *
  * If anything unusual happens, such as:
  *
  * - encountering a page which has buffers
  * - encountering a page which has a non-hole after a hole
  * - encountering a page with non-contiguous blocks
  *
  * then this code just gives up and calls the buffer_head-based read function.
  * It does handle a page which has holes at the end - that is a common case:
  * the end-of-file on blocksize < PAGE_SIZE setups.
  *
  * BH_Boundary explanation:
  *
  * There is a problem.  The mpage read code assembles several pages, gets all
  * their disk mappings, and then submits them all.  That's fine, but obtaining
  * the disk mappings may require I/O.  Reads of indirect blocks, for example.
  *
  * So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
  * submitted in the following order:
  *
  * 	12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
  *
  * because the indirect block has to be read to get the mappings of blocks
  * 13,14,15,16.  Obviously, this impacts performance.
  *
  * So what we do it to allow the filesystem's get_block() function to set
  * BH_Boundary when it maps block 11.  BH_Boundary says: mapping of the block
  * after this one will require I/O against a block which is probably close to
  * this one.  So you should push what I/O you have currently accumulated.
  *
  * This all causes the disk requests to be issued in the correct order.
  */
 void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
 {
 	struct page *page;
 	struct mpage_readpage_args args = {
 		.get_block = get_block,
 		.is_readahead = true,
 	};

 	while ((page = readahead_page(rac))) {
 		prefetchw(&page->flags);
 		args.page = page;
 		args.nr_pages = readahead_count(rac);
 		args.bio = do_mpage_readpage(&args);
 		put_page(page);
 	}
 	if (args.bio)
 		mpage_bio_submit(args.bio);
 }
 EXPORT_SYMBOL(mpage_readahead);

 /*
  * This isn't called much at all
  */
 int mpage_read_folio(struct folio *folio, get_block_t get_block)
 {
 	struct mpage_readpage_args args = {
 		.page = &folio->page,
 		.nr_pages = 1,
 		.get_block = get_block,
 	};

 	VM_BUG_ON_FOLIO(folio_test_large(folio), folio);

 	args.bio = do_mpage_readpage(&args);
 	if (args.bio)
 		mpage_bio_submit(args.bio);
 	return 0;
 }
 EXPORT_SYMBOL(mpage_read_folio);

 /*
  * Writing is not so simple.
  *
  * If the page has buffers then they will be used for obtaining the disk
  * mapping.  We only support pages which are fully mapped-and-dirty, with a
  * special case for pages which are unmapped at the end: end-of-file.
  *
  * If the page has no buffers (preferred) then the page is mapped here.
  *
  * If all blocks are found to be contiguous then the page can go into the
  * BIO.  Otherwise fall back to the mapping's writepage().
  *
  * FIXME: This code wants an estimate of how many pages are still to be
  * written, so it can intelligently allocate a suitably-sized BIO.  For now,
  * just allocate full-size (16-page) BIOs.
  */

 struct mpage_data {
 	struct bio *bio;
 	sector_t last_block_in_bio;
 	get_block_t *get_block;
 	unsigned use_writepage;
 };

 /*
  * We have our BIO, so we can now mark the buffers clean.  Make
  * sure to only clean buffers which we know we'll be writing.
  */
 static void clean_buffers(struct page *page, unsigned first_unmapped)
 {
 	unsigned buffer_counter = 0;
 	struct buffer_head *bh, *head;
 	if (!page_has_buffers(page))
 		return;
 	head = page_buffers(page);
 	bh = head;

 	do {
 		if (buffer_counter++ == first_unmapped)
 			break;
 		clear_buffer_dirty(bh);
 		bh = bh->b_this_page;
 	} while (bh != head);

 	/*
 	 * we cannot drop the bh if the page is not uptodate or a concurrent
 	 * read_folio would fail to serialize with the bh and it would read from
 	 * disk before we reach the platter.
 	 */
 	if (buffer_heads_over_limit && PageUptodate(page))
 		try_to_free_buffers(page_folio(page));
 }

 /*
  * For situations where we want to clean all buffers attached to a page.
  * We don't need to calculate how many buffers are attached to the page,
  * we just need to specify a number larger than the maximum number of buffers.
  */
 void clean_page_buffers(struct page *page)
 {
 	clean_buffers(page, ~0U);
 }

 static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 		      void *data)
 {
 	struct mpage_data *mpd = data;
 	struct bio *bio = mpd->bio;
 	struct address_space *mapping = page->mapping;
 	struct inode *inode = page->mapping->host;
 	const unsigned blkbits = inode->i_blkbits;
 	unsigned long end_index;
 	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
 	sector_t last_block;
 	sector_t block_in_file;
 	sector_t blocks[MAX_BUF_PER_PAGE];
 	unsigned page_block;
 	unsigned first_unmapped = blocks_per_page;
 	struct block_device *bdev = NULL;
 	int boundary = 0;
 	sector_t boundary_block = 0;
 	struct block_device *boundary_bdev = NULL;
 	int length;
 	struct buffer_head map_bh;
 	loff_t i_size = i_size_read(inode);
 	int ret = 0;

 	if (page_has_buffers(page)) {
 		struct buffer_head *head = page_buffers(page);
 		struct buffer_head *bh = head;

 		/* If they're all mapped and dirty, do it */
 		page_block = 0;
 		do {
 			BUG_ON(buffer_locked(bh));
 			if (!buffer_mapped(bh)) {
 				/*
 				 * unmapped dirty buffers are created by
 				 * block_dirty_folio -> mmapped data
 				 */
 				if (buffer_dirty(bh))
 					goto confused;
 				if (first_unmapped == blocks_per_page)
 					first_unmapped = page_block;
 				continue;
 			}

 			if (first_unmapped != blocks_per_page)
 				goto confused;	/* hole -> non-hole */

 			if (!buffer_dirty(bh) || !buffer_uptodate(bh))
 				goto confused;
 			if (page_block) {
 				if (bh->b_blocknr != blocks[page_block-1] + 1)
 					goto confused;
 			}
 			blocks[page_block++] = bh->b_blocknr;
 			boundary = buffer_boundary(bh);
 			if (boundary) {
 				boundary_block = bh->b_blocknr;
 				boundary_bdev = bh->b_bdev;
 			}
 			bdev = bh->b_bdev;
 		} while ((bh = bh->b_this_page) != head);

 		if (first_unmapped)
 			goto page_is_mapped;

 		/*
 		 * Page has buffers, but they are all unmapped. The page was
 		 * created by pagein or read over a hole which was handled by
 		 * block_read_full_folio().  If this address_space is also
 		 * using mpage_readahead then this can rarely happen.
 		 */
 		goto confused;
 	}

 	/*
 	 * The page has no buffers: map it to disk
 	 */
 	BUG_ON(!PageUptodate(page));
 	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
 	last_block = (i_size - 1) >> blkbits;
 	map_bh.b_page = page;
 	for (page_block = 0; page_block < blocks_per_page; ) {

 		map_bh.b_state = 0;
 		map_bh.b_size = 1 << blkbits;
 		if (mpd->get_block(inode, block_in_file, &map_bh, 1))
 			goto confused;
 		if (buffer_new(&map_bh))
 			clean_bdev_bh_alias(&map_bh);
 		if (buffer_boundary(&map_bh)) {
 			boundary_block = map_bh.b_blocknr;
 			boundary_bdev = map_bh.b_bdev;
 		}
 		if (page_block) {
 			if (map_bh.b_blocknr != blocks[page_block-1] + 1)
 				goto confused;
 		}
 		blocks[page_block++] = map_bh.b_blocknr;
 		boundary = buffer_boundary(&map_bh);
 		bdev = map_bh.b_bdev;
 		if (block_in_file == last_block)
 			break;
 		block_in_file++;
 	}
 	BUG_ON(page_block == 0);

 	first_unmapped = page_block;

 page_is_mapped:
 	end_index = i_size >> PAGE_SHIFT;
 	if (page->index >= end_index) {
 		/*
 		 * The page straddles i_size.  It must be zeroed out on each
 		 * and every writepage invocation because it may be mmapped.
 		 * "A file is mapped in multiples of the page size.  For a file
 		 * that is not a multiple of the page size, the remaining memory
 		 * is zeroed when mapped, and writes to that region are not
 		 * written out to the file."
 		 */
 		unsigned offset = i_size & (PAGE_SIZE - 1);

 		if (page->index > end_index || !offset)
 			goto confused;
 		zero_user_segment(page, offset, PAGE_SIZE);
 	}

 	/*
 	 * This page will go to BIO.  Do we need to send this BIO off first?
 	 */
 	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
 		bio = mpage_bio_submit(bio);

 alloc_new:
 	if (bio == NULL) {
 		if (first_unmapped == blocks_per_page) {
 			if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
 								page, wbc))
 				goto out;
 		}
 		bio = bio_alloc(bdev, BIO_MAX_VECS,
 				REQ_OP_WRITE | wbc_to_write_flags(wbc),
 				GFP_NOFS);
 		bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
 		wbc_init_bio(wbc, bio);
 	}

 	/*
 	 * Must try to add the page before marking the buffer clean or
 	 * the confused fail path above (OOM) will be very confused when
 	 * it finds all bh marked clean (i.e. it will not write anything)
 	 */
 	wbc_account_cgroup_owner(wbc, page, PAGE_SIZE);
 	length = first_unmapped << blkbits;
 	if (bio_add_page(bio, page, length, 0) < length) {
 		bio = mpage_bio_submit(bio);
 		goto alloc_new;
 	}

 	clean_buffers(page, first_unmapped);

 	BUG_ON(PageWriteback(page));
 	set_page_writeback(page);
 	unlock_page(page);
 	if (boundary || (first_unmapped != blocks_per_page)) {
 		bio = mpage_bio_submit(bio);
 		if (boundary_block) {
 			write_boundary_block(boundary_bdev,
 					boundary_block, 1 << blkbits);
 		}
 	} else {
 		mpd->last_block_in_bio = blocks[blocks_per_page - 1];
 	}
 	goto out;

 confused:
 	if (bio)
 		bio = mpage_bio_submit(bio);

 	if (mpd->use_writepage) {
 		ret = mapping->a_ops->writepage(page, wbc);
 	} else {
 		ret = -EAGAIN;
 		goto out;
 	}
 	/*
 	 * The caller has a ref on the inode, so *mapping is stable
 	 */
 	mapping_set_error(mapping, ret);
 out:
 	mpd->bio = bio;
 	return ret;
 }

 /**
  * mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
  * @mapping: address space structure to write
  * @wbc: subtract the number of written pages from *@wbc->nr_to_write
  * @get_block: the filesystem's block mapper function.
  *             If this is NULL then use a_ops->writepage.  Otherwise, go
  *             direct-to-BIO.
  *
  * This is a library function, which implements the writepages()
  * address_space_operation.
  *
  * If a page is already under I/O, generic_writepages() skips it, even
  * if it's dirty.  This is desirable behaviour for memory-cleaning writeback,
  * but it is INCORRECT for data-integrity system calls such as fsync().  fsync()
  * and msync() need to guarantee that all the data which was dirty at the time
  * the call was made get new I/O started against them.  If wbc->sync_mode is
  * WB_SYNC_ALL then we were called for data integrity and we must wait for
  * existing IO to complete.
  */
 int
 mpage_writepages(struct address_space *mapping,
 		struct writeback_control *wbc, get_block_t get_block)
 {
 	struct blk_plug plug;
 	int ret;

 	blk_start_plug(&plug);

 	if (!get_block)
 		ret = generic_writepages(mapping, wbc);
 	else {
 		struct mpage_data mpd = {
 			.bio = NULL,
 			.last_block_in_bio = 0,
 			.get_block = get_block,
 			.use_writepage = 1,
 		};

 		ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
 		if (mpd.bio)
 			mpage_bio_submit(mpd.bio);
 	}
 	blk_finish_plug(&plug);
 	return ret;
 }
 EXPORT_SYMBOL(mpage_writepages);

 int mpage_writepage(struct page *page, get_block_t get_block,
 	struct writeback_control *wbc)
 {
 	struct mpage_data mpd = {
 		.bio = NULL,
 		.last_block_in_bio = 0,
 		.get_block = get_block,
 		.use_writepage = 0,
 	};
 	int ret = __mpage_writepage(page, wbc, &mpd);
 	if (mpd.bio)
 		mpage_bio_submit(mpd.bio);
 	return ret;
 }
 EXPORT_SYMBOL(mpage_writepage);
	// SPDX-License-Identifier: GPL-2.0
	/*
	* fs/mpage.c
	*
	* Copyright (C) 2002, Linus Torvalds.
	*
	* Contains functions related to preparing and submitting BIOs which contain
	* multiple pagecache pages.
	*
	* 15May2002 Andrew Morton
	* Initial version
	* 27Jun2002 axboe@suse.de
	* use bio_add_page() to build bio's just the right size
	*/

	#include <linux/kernel.h>
	#include <linux/export.h>
	#include <linux/mm.h>
	#include <linux/kdev_t.h>
	#include <linux/gfp.h>
	#include <linux/bio.h>
	#include <linux/fs.h>
	#include <linux/buffer_head.h>
	#include <linux/blkdev.h>
	#include <linux/highmem.h>
	#include <linux/prefetch.h>
	#include <linux/mpage.h>
	#include <linux/mm_inline.h>
	#include <linux/writeback.h>
	#include <linux/backing-dev.h>
	#include <linux/pagevec.h>
	#include "internal.h"

	/*
	* I/O completion handler for multipage BIOs.
	*
	* The mpage code never puts partial pages into a BIO (except for end-of-file).
	* If a page does not map to a contiguous run of blocks then it simply falls
	* back to block_read_full_folio().
	*
	* Why is this? If a page's completion depends on a number of different BIOs
	* which can complete in any order (or at the same time) then determining the
	* status of that page is hard. See end_buffer_async_read() for the details.
	* There is no point in duplicating all that complexity.
	*/
	static void mpage_end_io(struct bio *bio)
	{
	struct bio_vec *bv;
	struct bvec_iter_all iter_all;

	bio_for_each_segment_all(bv, bio, iter_all) {
	struct page *page = bv->bv_page;
	page_endio(page, bio_op(bio),
	blk_status_to_errno(bio->bi_status));
	}

	bio_put(bio);
	}

	static struct bio mpage_bio_submit(struct bio bio)
	{
	bio->bi_end_io = mpage_end_io;
	guard_bio_eod(bio);
	submit_bio(bio);
	return NULL;
	}

	/*
	* support function for mpage_readahead. The fs supplied get_block might
	* return an up to date buffer. This is used to map that buffer into
	* the page, which allows read_folio to avoid triggering a duplicate call
	* to get_block.
	*
	* The idea is to avoid adding buffers to pages that don't already have
	* them. So when the buffer is up to date and the page size == block size,
	* this marks the page up to date instead of adding new buffers.
	*/
	static void
	map_buffer_to_page(struct page page, struct buffer_head bh, int page_block)
	{
	struct inode *inode = page->mapping->host;
	struct buffer_head page_bh, head;
	int block = 0;

	if (!page_has_buffers(page)) {
	/*
	* don't make any buffers if there is only one buffer on
	* the page and the page just needs to be set up to date
	*/
	if (inode->i_blkbits == PAGE_SHIFT &&
	buffer_uptodate(bh)) {
	SetPageUptodate(page);
	return;
	}
	create_empty_buffers(page, i_blocksize(inode), 0);
	}
	head = page_buffers(page);
	page_bh = head;
	do {
	if (block == page_block) {
	page_bh->b_state = bh->b_state;
	page_bh->b_bdev = bh->b_bdev;
	page_bh->b_blocknr = bh->b_blocknr;
	break;
	}
	page_bh = page_bh->b_this_page;
	block++;
	} while (page_bh != head);
	}

	struct mpage_readpage_args {
	struct bio *bio;
	struct page *page;
	unsigned int nr_pages;
	bool is_readahead;
	sector_t last_block_in_bio;
	struct buffer_head map_bh;
	unsigned long first_logical_block;
	get_block_t *get_block;
	};

	/*
	* This is the worker routine which does all the work of mapping the disk
	* blocks and constructs largest possible bios, submits them for IO if the
	* blocks are not contiguous on the disk.
	*
	* We pass a buffer_head back and forth and use its buffer_mapped() flag to
	* represent the validity of its disk mapping and to decide when to do the next
	* get_block() call.
	*/
	static struct bio do_mpage_readpage(struct mpage_readpage_args args)
	{
	struct page *page = args->page;
	struct inode *inode = page->mapping->host;
	const unsigned blkbits = inode->i_blkbits;
	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
	const unsigned blocksize = 1 << blkbits;
	struct buffer_head *map_bh = &args->map_bh;
	sector_t block_in_file;
	sector_t last_block;
	sector_t last_block_in_file;
	sector_t blocks[MAX_BUF_PER_PAGE];
	unsigned page_block;
	unsigned first_hole = blocks_per_page;
	struct block_device *bdev = NULL;
	int length;
	int fully_mapped = 1;
	int op = REQ_OP_READ;
	unsigned nblocks;
	unsigned relative_block;
	gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL);

	if (args->is_readahead) {
	op \|= REQ_RAHEAD;
	gfp \|= __GFP_NORETRY \| __GFP_NOWARN;
	}

	if (page_has_buffers(page))
	goto confused;

	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
	last_block = block_in_file + args->nr_pages * blocks_per_page;
	last_block_in_file = (i_size_read(inode) + blocksize - 1) >> blkbits;
	if (last_block > last_block_in_file)
	last_block = last_block_in_file;
	page_block = 0;

	/*
	* Map blocks using the result from the previous get_blocks call first.
	*/
	nblocks = map_bh->b_size >> blkbits;
	if (buffer_mapped(map_bh) &&
	block_in_file > args->first_logical_block &&
	block_in_file < (args->first_logical_block + nblocks)) {
	unsigned map_offset = block_in_file - args->first_logical_block;
	unsigned last = nblocks - map_offset;

	for (relative_block = 0; ; relative_block++) {
	if (relative_block == last) {
	clear_buffer_mapped(map_bh);
	break;
	}
	if (page_block == blocks_per_page)
	break;
	blocks[page_block] = map_bh->b_blocknr + map_offset +
	relative_block;
	page_block++;
	block_in_file++;
	}
	bdev = map_bh->b_bdev;
	}

	/*
	* Then do more get_blocks calls until we are done with this page.
	*/
	map_bh->b_page = page;
	while (page_block < blocks_per_page) {
	map_bh->b_state = 0;
	map_bh->b_size = 0;

	if (block_in_file < last_block) {
	map_bh->b_size = (last_block-block_in_file) << blkbits;
	if (args->get_block(inode, block_in_file, map_bh, 0))
	goto confused;
	args->first_logical_block = block_in_file;
	}

	if (!buffer_mapped(map_bh)) {
	fully_mapped = 0;
	if (first_hole == blocks_per_page)
	first_hole = page_block;
	page_block++;
	block_in_file++;
	continue;
	}

	/* some filesystems will copy data into the page during
	* the get_block call, in which case we don't want to
	* read it again. map_buffer_to_page copies the data
	* we just collected from get_block into the page's buffers
	* so readpage doesn't have to repeat the get_block call
	*/
	if (buffer_uptodate(map_bh)) {
	map_buffer_to_page(page, map_bh, page_block);
	goto confused;
	}

	if (first_hole != blocks_per_page)
	goto confused; /* hole -> non-hole */

	/* Contiguous blocks? */
	if (page_block && blocks[page_block-1] != map_bh->b_blocknr-1)
	goto confused;
	nblocks = map_bh->b_size >> blkbits;
	for (relative_block = 0; ; relative_block++) {
	if (relative_block == nblocks) {
	clear_buffer_mapped(map_bh);
	break;
	} else if (page_block == blocks_per_page)
	break;
	blocks[page_block] = map_bh->b_blocknr+relative_block;
	page_block++;
	block_in_file++;
	}
	bdev = map_bh->b_bdev;
	}

	if (first_hole != blocks_per_page) {
	zero_user_segment(page, first_hole << blkbits, PAGE_SIZE);
	if (first_hole == 0) {
	SetPageUptodate(page);
	unlock_page(page);
	goto out;
	}
	} else if (fully_mapped) {
	SetPageMappedToDisk(page);
	}

	/*
	* This page will go to BIO. Do we need to send this BIO off first?
	*/
	if (args->bio && (args->last_block_in_bio != blocks[0] - 1))
	args->bio = mpage_bio_submit(args->bio);

	alloc_new:
	if (args->bio == NULL) {
	if (first_hole == blocks_per_page) {
	if (!bdev_read_page(bdev, blocks[0] << (blkbits - 9),
	page))
	goto out;
	}
	args->bio = bio_alloc(bdev, bio_max_segs(args->nr_pages), op,
	gfp);
	if (args->bio == NULL)
	goto confused;
	args->bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
	}

	length = first_hole << blkbits;
	if (bio_add_page(args->bio, page, length, 0) < length) {
	args->bio = mpage_bio_submit(args->bio);
	goto alloc_new;
	}

	relative_block = block_in_file - args->first_logical_block;
	nblocks = map_bh->b_size >> blkbits;
	if ((buffer_boundary(map_bh) && relative_block == nblocks) \|\|
	(first_hole != blocks_per_page))
	args->bio = mpage_bio_submit(args->bio);
	else
	args->last_block_in_bio = blocks[blocks_per_page - 1];
	out:
	return args->bio;

	confused:
	if (args->bio)
	args->bio = mpage_bio_submit(args->bio);
	if (!PageUptodate(page))
	block_read_full_folio(page_folio(page), args->get_block);
	else
	unlock_page(page);
	goto out;
	}

	/**
	* mpage_readahead - start reads against pages
	* @rac: Describes which pages to read.
	* @get_block: The filesystem's block mapper function.
	*
	* This function walks the pages and the blocks within each page, building and
	* emitting large BIOs.
	*
	* If anything unusual happens, such as:
	*
	* - encountering a page which has buffers
	* - encountering a page which has a non-hole after a hole
	* - encountering a page with non-contiguous blocks
	*
	* then this code just gives up and calls the buffer_head-based read function.
	* It does handle a page which has holes at the end - that is a common case:
	* the end-of-file on blocksize < PAGE_SIZE setups.
	*
	* BH_Boundary explanation:
	*
	* There is a problem. The mpage read code assembles several pages, gets all
	* their disk mappings, and then submits them all. That's fine, but obtaining
	* the disk mappings may require I/O. Reads of indirect blocks, for example.
	*
	* So an mpage read of the first 16 blocks of an ext2 file will cause I/O to be
	* submitted in the following order:
	*
	* 12 0 1 2 3 4 5 6 7 8 9 10 11 13 14 15 16
	*
	* because the indirect block has to be read to get the mappings of blocks
	* 13,14,15,16. Obviously, this impacts performance.
	*
	* So what we do it to allow the filesystem's get_block() function to set
	* BH_Boundary when it maps block 11. BH_Boundary says: mapping of the block
	* after this one will require I/O against a block which is probably close to
	* this one. So you should push what I/O you have currently accumulated.
	*
	* This all causes the disk requests to be issued in the correct order.
	*/
	void mpage_readahead(struct readahead_control *rac, get_block_t get_block)
	{
	struct page *page;
	struct mpage_readpage_args args = {
	.get_block = get_block,
	.is_readahead = true,
	};

	while ((page = readahead_page(rac))) {
	prefetchw(&page->flags);
	args.page = page;
	args.nr_pages = readahead_count(rac);
	args.bio = do_mpage_readpage(&args);
	put_page(page);
	}
	if (args.bio)
	mpage_bio_submit(args.bio);
	}
	EXPORT_SYMBOL(mpage_readahead);

	/*
	* This isn't called much at all
	*/
	int mpage_read_folio(struct folio *folio, get_block_t get_block)
	{
	struct mpage_readpage_args args = {
	.page = &folio->page,
	.nr_pages = 1,
	.get_block = get_block,
	};

	VM_BUG_ON_FOLIO(folio_test_large(folio), folio);

	args.bio = do_mpage_readpage(&args);
	if (args.bio)
	mpage_bio_submit(args.bio);
	return 0;
	}
	EXPORT_SYMBOL(mpage_read_folio);

	/*
	* Writing is not so simple.
	*
	* If the page has buffers then they will be used for obtaining the disk
	* mapping. We only support pages which are fully mapped-and-dirty, with a
	* special case for pages which are unmapped at the end: end-of-file.
	*
	* If the page has no buffers (preferred) then the page is mapped here.
	*
	* If all blocks are found to be contiguous then the page can go into the
	* BIO. Otherwise fall back to the mapping's writepage().
	*
	* FIXME: This code wants an estimate of how many pages are still to be
	* written, so it can intelligently allocate a suitably-sized BIO. For now,
	* just allocate full-size (16-page) BIOs.
	*/

	struct mpage_data {
	struct bio *bio;
	sector_t last_block_in_bio;
	get_block_t *get_block;
	unsigned use_writepage;
	};

	/*
	* We have our BIO, so we can now mark the buffers clean. Make
	* sure to only clean buffers which we know we'll be writing.
	*/
	static void clean_buffers(struct page *page, unsigned first_unmapped)
	{
	unsigned buffer_counter = 0;
	struct buffer_head bh, head;
	if (!page_has_buffers(page))
	return;
	head = page_buffers(page);
	bh = head;

	do {
	if (buffer_counter++ == first_unmapped)
	break;
	clear_buffer_dirty(bh);
	bh = bh->b_this_page;
	} while (bh != head);

	/*
	* we cannot drop the bh if the page is not uptodate or a concurrent
	* read_folio would fail to serialize with the bh and it would read from
	* disk before we reach the platter.
	*/
	if (buffer_heads_over_limit && PageUptodate(page))
	try_to_free_buffers(page_folio(page));
	}

	/*
	* For situations where we want to clean all buffers attached to a page.
	* We don't need to calculate how many buffers are attached to the page,
	* we just need to specify a number larger than the maximum number of buffers.
	*/
	void clean_page_buffers(struct page *page)
	{
	clean_buffers(page, ~0U);
	}

	static int __mpage_writepage(struct page page, struct writeback_control wbc,
	void *data)
	{
	struct mpage_data *mpd = data;
	struct bio *bio = mpd->bio;
	struct address_space *mapping = page->mapping;
	struct inode *inode = page->mapping->host;
	const unsigned blkbits = inode->i_blkbits;
	unsigned long end_index;
	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
	sector_t last_block;
	sector_t block_in_file;
	sector_t blocks[MAX_BUF_PER_PAGE];
	unsigned page_block;
	unsigned first_unmapped = blocks_per_page;
	struct block_device *bdev = NULL;
	int boundary = 0;
	sector_t boundary_block = 0;
	struct block_device *boundary_bdev = NULL;
	int length;
	struct buffer_head map_bh;
	loff_t i_size = i_size_read(inode);
	int ret = 0;

	if (page_has_buffers(page)) {
	struct buffer_head *head = page_buffers(page);
	struct buffer_head *bh = head;

	/* If they're all mapped and dirty, do it */
	page_block = 0;
	do {
	BUG_ON(buffer_locked(bh));
	if (!buffer_mapped(bh)) {
	/*
	* unmapped dirty buffers are created by
	* block_dirty_folio -> mmapped data
	*/
	if (buffer_dirty(bh))
	goto confused;
	if (first_unmapped == blocks_per_page)
	first_unmapped = page_block;
	continue;
	}

	if (first_unmapped != blocks_per_page)
	goto confused; /* hole -> non-hole */

	if (!buffer_dirty(bh) \|\| !buffer_uptodate(bh))
	goto confused;
	if (page_block) {
	if (bh->b_blocknr != blocks[page_block-1] + 1)
	goto confused;
	}
	blocks[page_block++] = bh->b_blocknr;
	boundary = buffer_boundary(bh);
	if (boundary) {
	boundary_block = bh->b_blocknr;
	boundary_bdev = bh->b_bdev;
	}
	bdev = bh->b_bdev;
	} while ((bh = bh->b_this_page) != head);

	if (first_unmapped)
	goto page_is_mapped;

	/*
	* Page has buffers, but they are all unmapped. The page was
	* created by pagein or read over a hole which was handled by
	* block_read_full_folio(). If this address_space is also
	* using mpage_readahead then this can rarely happen.
	*/
	goto confused;
	}

	/*
	* The page has no buffers: map it to disk
	*/
	BUG_ON(!PageUptodate(page));
	block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
	last_block = (i_size - 1) >> blkbits;
	map_bh.b_page = page;
	for (page_block = 0; page_block < blocks_per_page; ) {

	map_bh.b_state = 0;
	map_bh.b_size = 1 << blkbits;
	if (mpd->get_block(inode, block_in_file, &map_bh, 1))
	goto confused;
	if (buffer_new(&map_bh))
	clean_bdev_bh_alias(&map_bh);
	if (buffer_boundary(&map_bh)) {
	boundary_block = map_bh.b_blocknr;
	boundary_bdev = map_bh.b_bdev;
	}
	if (page_block) {
	if (map_bh.b_blocknr != blocks[page_block-1] + 1)
	goto confused;
	}
	blocks[page_block++] = map_bh.b_blocknr;
	boundary = buffer_boundary(&map_bh);
	bdev = map_bh.b_bdev;
	if (block_in_file == last_block)
	break;
	block_in_file++;
	}
	BUG_ON(page_block == 0);

	first_unmapped = page_block;

	page_is_mapped:
	end_index = i_size >> PAGE_SHIFT;
	if (page->index >= end_index) {
	/*
	* The page straddles i_size. It must be zeroed out on each
	* and every writepage invocation because it may be mmapped.
	* "A file is mapped in multiples of the page size. For a file
	* that is not a multiple of the page size, the remaining memory
	* is zeroed when mapped, and writes to that region are not
	* written out to the file."
	*/
	unsigned offset = i_size & (PAGE_SIZE - 1);

	if (page->index > end_index \|\| !offset)
	goto confused;
	zero_user_segment(page, offset, PAGE_SIZE);
	}

	/*
	* This page will go to BIO. Do we need to send this BIO off first?
	*/
	if (bio && mpd->last_block_in_bio != blocks[0] - 1)
	bio = mpage_bio_submit(bio);

	alloc_new:
	if (bio == NULL) {
	if (first_unmapped == blocks_per_page) {
	if (!bdev_write_page(bdev, blocks[0] << (blkbits - 9),
	page, wbc))
	goto out;
	}
	bio = bio_alloc(bdev, BIO_MAX_VECS,
	REQ_OP_WRITE \| wbc_to_write_flags(wbc),
	GFP_NOFS);
	bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
	wbc_init_bio(wbc, bio);
	}

	/*
	* Must try to add the page before marking the buffer clean or
	* the confused fail path above (OOM) will be very confused when
	* it finds all bh marked clean (i.e. it will not write anything)
	*/
	wbc_account_cgroup_owner(wbc, page, PAGE_SIZE);
	length = first_unmapped << blkbits;
	if (bio_add_page(bio, page, length, 0) < length) {
	bio = mpage_bio_submit(bio);
	goto alloc_new;
	}

	clean_buffers(page, first_unmapped);

	BUG_ON(PageWriteback(page));
	set_page_writeback(page);
	unlock_page(page);
	if (boundary \|\| (first_unmapped != blocks_per_page)) {
	bio = mpage_bio_submit(bio);
	if (boundary_block) {
	write_boundary_block(boundary_bdev,
	boundary_block, 1 << blkbits);
	}
	} else {
	mpd->last_block_in_bio = blocks[blocks_per_page - 1];
	}
	goto out;

	confused:
	if (bio)
	bio = mpage_bio_submit(bio);

	if (mpd->use_writepage) {
	ret = mapping->a_ops->writepage(page, wbc);
	} else {
	ret = -EAGAIN;
	goto out;
	}
	/*
	* The caller has a ref on the inode, so *mapping is stable
	*/
	mapping_set_error(mapping, ret);
	out:
	mpd->bio = bio;
	return ret;
	}

	/**
	* mpage_writepages - walk the list of dirty pages of the given address space & writepage() all of them
	* @mapping: address space structure to write
	* @wbc: subtract the number of written pages from *@wbc->nr_to_write
	* @get_block: the filesystem's block mapper function.
	* If this is NULL then use a_ops->writepage. Otherwise, go
	* direct-to-BIO.
	*
	* This is a library function, which implements the writepages()
	* address_space_operation.
	*
	* If a page is already under I/O, generic_writepages() skips it, even
	* if it's dirty. This is desirable behaviour for memory-cleaning writeback,
	* but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
	* and msync() need to guarantee that all the data which was dirty at the time
	* the call was made get new I/O started against them. If wbc->sync_mode is
	* WB_SYNC_ALL then we were called for data integrity and we must wait for
	* existing IO to complete.
	*/
	int
	mpage_writepages(struct address_space *mapping,
	struct writeback_control *wbc, get_block_t get_block)
	{
	struct blk_plug plug;
	int ret;

	blk_start_plug(&plug);

	if (!get_block)
	ret = generic_writepages(mapping, wbc);
	else {
	struct mpage_data mpd = {
	.bio = NULL,
	.last_block_in_bio = 0,
	.get_block = get_block,
	.use_writepage = 1,
	};

	ret = write_cache_pages(mapping, wbc, __mpage_writepage, &mpd);
	if (mpd.bio)
	mpage_bio_submit(mpd.bio);
	}
	blk_finish_plug(&plug);
	return ret;
	}
	EXPORT_SYMBOL(mpage_writepages);

	int mpage_writepage(struct page *page, get_block_t get_block,
	struct writeback_control *wbc)
	{
	struct mpage_data mpd = {
	.bio = NULL,
	.last_block_in_bio = 0,
	.get_block = get_block,
	.use_writepage = 0,
	};
	int ret = __mpage_writepage(page, wbc, &mpd);
	if (mpd.bio)
	mpage_bio_submit(mpd.bio);
	return ret;
	}
	EXPORT_SYMBOL(mpage_writepage);