| /* |
| * mm/readahead.c - address_space-level file readahead. |
| * |
| * Copyright (C) 2002, Linus Torvalds |
| * |
| * 09Apr2002 akpm@zip.com.au |
| * Initial version. |
| */ |
| |
| #include <linux/kernel.h> |
| #include <linux/fs.h> |
| #include <linux/mm.h> |
| #include <linux/blkdev.h> |
| #include <linux/backing-dev.h> |
| #include <linux/pagevec.h> |
| |
| struct backing_dev_info default_backing_dev_info = { |
| .ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE, |
| .state = 0, |
| }; |
| |
| /* |
| * Initialise a struct file's readahead state |
| */ |
| void |
| file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping) |
| { |
| memset(ra, 0, sizeof(*ra)); |
| ra->ra_pages = mapping->backing_dev_info->ra_pages; |
| } |
| |
| /* |
| * Return max readahead size for this inode in number-of-pages. |
| */ |
| static inline unsigned long get_max_readahead(struct file_ra_state *ra) |
| { |
| return ra->ra_pages; |
| } |
| |
| static inline unsigned long get_min_readahead(struct file_ra_state *ra) |
| { |
| return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE; |
| } |
| |
| /** |
| * read_cache_pages - populate an address space with some pages, and |
| * start reads against them. |
| * @mapping: the address_space |
| * @pages: The address of a list_head which contains the target pages. These |
| * pages have their ->index populated and are otherwise uninitialised. |
| * @filler: callback routine for filling a single page. |
| * @data: private data for the callback routine. |
| * |
| * Hides the details of the LRU cache etc from the filesystems. |
| */ |
| int read_cache_pages(struct address_space *mapping, struct list_head *pages, |
| int (*filler)(void *, struct page *), void *data) |
| { |
| struct page *page; |
| struct pagevec lru_pvec; |
| int ret = 0; |
| |
| pagevec_init(&lru_pvec, 0); |
| |
| while (!list_empty(pages)) { |
| page = list_entry(pages->prev, struct page, list); |
| list_del(&page->list); |
| if (add_to_page_cache(page, mapping, page->index, GFP_KERNEL)) { |
| page_cache_release(page); |
| continue; |
| } |
| ret = filler(data, page); |
| if (!pagevec_add(&lru_pvec, page)) |
| __pagevec_lru_add(&lru_pvec); |
| if (ret) |
| break; |
| } |
| pagevec_lru_add(&lru_pvec); |
| return ret; |
| } |
| |
| static int read_pages(struct address_space *mapping, struct file *filp, |
| struct list_head *pages, unsigned nr_pages) |
| { |
| unsigned page_idx; |
| struct pagevec lru_pvec; |
| |
| pagevec_init(&lru_pvec, 0); |
| |
| if (mapping->a_ops->readpages) |
| return mapping->a_ops->readpages(filp, mapping, pages, nr_pages); |
| |
| for (page_idx = 0; page_idx < nr_pages; page_idx++) { |
| struct page *page = list_entry(pages->prev, struct page, list); |
| list_del(&page->list); |
| if (!add_to_page_cache(page, mapping, |
| page->index, GFP_KERNEL)) { |
| mapping->a_ops->readpage(filp, page); |
| if (!pagevec_add(&lru_pvec, page)) |
| __pagevec_lru_add(&lru_pvec); |
| } else { |
| page_cache_release(page); |
| } |
| } |
| pagevec_lru_add(&lru_pvec); |
| return 0; |
| } |
| |
| /* |
| * Readahead design. |
| * |
| * The fields in struct file_ra_state represent the most-recently-executed |
| * readahead attempt: |
| * |
| * start: Page index at which we started the readahead |
| * size: Number of pages in that read |
| * Together, these form the "current window". |
| * Together, start and size represent the `readahead window'. |
| * next_size: The number of pages to read on the next readahead miss. |
| * Has the magical value -1UL if readahead has been disabled. |
| * prev_page: The page which the readahead algorithm most-recently inspected. |
| * prev_page is mainly an optimisation: if page_cache_readahead |
| * sees that it is again being called for a page which it just |
| * looked at, it can return immediately without making any state |
| * changes. |
| * ahead_start, |
| * ahead_size: Together, these form the "ahead window". |
| * ra_pages: The externally controlled max readahead for this fd. |
| * |
| * The readahead code manages two windows - the "current" and the "ahead" |
| * windows. The intent is that while the application is walking the pages |
| * in the current window, I/O is underway on the ahead window. When the |
| * current window is fully traversed, it is replaced by the ahead window |
| * and the ahead window is invalidated. When this copying happens, the |
| * new current window's pages are probably still locked. When I/O has |
| * completed, we submit a new batch of I/O, creating a new ahead window. |
| * |
| * So: |
| * |
| * ----|----------------|----------------|----- |
| * ^start ^start+size |
| * ^ahead_start ^ahead_start+ahead_size |
| * |
| * ^ When this page is read, we submit I/O for the |
| * ahead window. |
| * |
| * A `readahead hit' occurs when a read request is made against a page which is |
| * inside the current window. Hits are good, and the window size (next_size) |
| * is grown aggressively when hits occur. Two pages are added to the next |
| * window size on each hit, which will end up doubling the next window size by |
| * the time I/O is submitted for it. |
| * |
| * If readahead hits are more sparse (say, the application is only reading |
| * every second page) then the window will build more slowly. |
| * |
| * On a readahead miss (the application seeked away) the readahead window is |
| * shrunk by 25%. We don't want to drop it too aggressively, because it is a |
| * good assumption that an application which has built a good readahead window |
| * will continue to perform linear reads. Either at the new file position, or |
| * at the old one after another seek. |
| * |
| * There is a special-case: if the first page which the application tries to |
| * read happens to be the first page of the file, it is assumed that a linear |
| * read is about to happen and the window is immediately set to half of the |
| * device maximum. |
| * |
| * A page request at (start + size) is not a miss at all - it's just a part of |
| * sequential file reading. |
| * |
| * This function is to be called for every page which is read, rather than when |
| * it is time to perform readahead. This is so the readahead algorithm can |
| * centrally work out the access patterns. This could be costly with many tiny |
| * read()s, so we specifically optimise for that case with prev_page. |
| */ |
| |
| /* |
| * do_page_cache_readahead actually reads a chunk of disk. It allocates all |
| * the pages first, then submits them all for I/O. This avoids the very bad |
| * behaviour which would occur if page allocations are causing VM writeback. |
| * We really don't want to intermingle reads and writes like that. |
| * |
| * Returns the number of pages which actually had IO started against them. |
| */ |
| static inline int |
| __do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
| unsigned long offset, unsigned long nr_to_read) |
| { |
| struct inode *inode = mapping->host; |
| struct page *page; |
| unsigned long end_index; /* The last page we want to read */ |
| LIST_HEAD(page_pool); |
| int page_idx; |
| int ret = 0; |
| |
| if (inode->i_size == 0) |
| goto out; |
| |
| end_index = ((inode->i_size - 1) >> PAGE_CACHE_SHIFT); |
| |
| /* |
| * Preallocate as many pages as we will need. |
| */ |
| read_lock(&mapping->page_lock); |
| for (page_idx = 0; page_idx < nr_to_read; page_idx++) { |
| unsigned long page_offset = offset + page_idx; |
| |
| if (page_offset > end_index) |
| break; |
| |
| page = radix_tree_lookup(&mapping->page_tree, page_offset); |
| if (page) |
| continue; |
| |
| read_unlock(&mapping->page_lock); |
| page = page_cache_alloc_cold(mapping); |
| read_lock(&mapping->page_lock); |
| if (!page) |
| break; |
| page->index = page_offset; |
| list_add(&page->list, &page_pool); |
| ret++; |
| } |
| read_unlock(&mapping->page_lock); |
| |
| /* |
| * Now start the IO. We ignore I/O errors - if the page is not |
| * uptodate then the caller will launch readpage again, and |
| * will then handle the error. |
| */ |
| if (ret) { |
| read_pages(mapping, filp, &page_pool, ret); |
| blk_run_queues(); |
| } |
| BUG_ON(!list_empty(&page_pool)); |
| out: |
| return ret; |
| } |
| |
| /* |
| * Chunk the readahead into 2 megabyte units, so that we don't pin too much |
| * memory at once. |
| */ |
| int do_page_cache_readahead(struct address_space *mapping, struct file *filp, |
| unsigned long offset, unsigned long nr_to_read) |
| { |
| int ret = 0; |
| |
| while (nr_to_read) { |
| unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_CACHE_SIZE; |
| |
| if (this_chunk > nr_to_read) |
| this_chunk = nr_to_read; |
| ret = __do_page_cache_readahead(mapping, filp, |
| offset, this_chunk); |
| if (ret < 0) |
| break; |
| offset += this_chunk; |
| nr_to_read -= this_chunk; |
| } |
| return ret; |
| } |
| |
| /* |
| * Check how effective readahead is being. If the amount of started IO is |
| * less than expected then the file is partly or fully in pagecache and |
| * readahead isn't helping. Shrink the window. |
| * |
| * But don't shrink it too much - the application may read the same page |
| * occasionally. |
| */ |
| static inline void |
| check_ra_success(struct file_ra_state *ra, pgoff_t attempt, |
| pgoff_t actual, pgoff_t orig_next_size) |
| { |
| if (actual == 0) { |
| if (orig_next_size > 1) { |
| ra->next_size = orig_next_size - 1; |
| if (ra->ahead_size) |
| ra->ahead_size = ra->next_size; |
| } else { |
| ra->next_size = -1UL; |
| } |
| } |
| } |
| |
| /* |
| * page_cache_readahead is the main function. If performs the adaptive |
| * readahead window size management and submits the readahead I/O. |
| */ |
| void |
| page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra, |
| struct file *filp, unsigned long offset) |
| { |
| unsigned max; |
| unsigned min; |
| unsigned orig_next_size; |
| unsigned actual; |
| |
| /* |
| * Here we detect the case where the application is performing |
| * sub-page sized reads. We avoid doing extra work and bogusly |
| * perturbing the readahead window expansion logic. |
| * If next_size is zero, this is the very first read for this |
| * file handle, or the window is maximally shrunk. |
| */ |
| if (offset == ra->prev_page) { |
| if (ra->next_size != 0) |
| goto out; |
| } |
| |
| if (ra->next_size == -1UL) |
| goto out; /* Maximally shrunk */ |
| |
| max = get_max_readahead(ra); |
| if (max == 0) |
| goto out; /* No readahead */ |
| |
| min = get_min_readahead(ra); |
| orig_next_size = ra->next_size; |
| |
| if (ra->next_size == 0 && offset == 0) { |
| /* |
| * Special case - first read from first page. |
| * We'll assume it's a whole-file read, and |
| * grow the window fast. |
| */ |
| ra->next_size = max / 2; |
| goto do_io; |
| } |
| |
| ra->prev_page = offset; |
| |
| if (offset >= ra->start && offset <= (ra->start + ra->size)) { |
| /* |
| * A readahead hit. Either inside the window, or one |
| * page beyond the end. Expand the next readahead size. |
| */ |
| ra->next_size += 2; |
| } else { |
| /* |
| * A miss - lseek, pread, etc. Shrink the readahead |
| * window by 25%. |
| */ |
| ra->next_size -= ra->next_size / 4; |
| } |
| |
| if (ra->next_size > max) |
| ra->next_size = max; |
| if (ra->next_size < min) |
| ra->next_size = min; |
| |
| /* |
| * Is this request outside the current window? |
| */ |
| if (offset < ra->start || offset >= (ra->start + ra->size)) { |
| /* |
| * A miss against the current window. Have we merely |
| * advanced into the ahead window? |
| */ |
| if (offset == ra->ahead_start) { |
| /* |
| * Yes, we have. The ahead window now becomes |
| * the current window. |
| */ |
| ra->start = ra->ahead_start; |
| ra->size = ra->ahead_size; |
| ra->prev_page = ra->start; |
| ra->ahead_start = 0; |
| ra->ahead_size = 0; |
| /* |
| * Control now returns, probably to sleep until I/O |
| * completes against the first ahead page. |
| * When the second page in the old ahead window is |
| * requested, control will return here and more I/O |
| * will be submitted to build the new ahead window. |
| */ |
| goto out; |
| } |
| do_io: |
| /* |
| * This is the "unusual" path. We come here during |
| * startup or after an lseek. We invalidate the |
| * ahead window and get some I/O underway for the new |
| * current window. |
| */ |
| ra->start = offset; |
| ra->size = ra->next_size; |
| ra->ahead_start = 0; /* Invalidate these */ |
| ra->ahead_size = 0; |
| |
| actual = do_page_cache_readahead(mapping, filp, offset, |
| ra->size); |
| check_ra_success(ra, ra->size, actual, orig_next_size); |
| } else { |
| /* |
| * This read request is within the current window. It is time |
| * to submit I/O for the ahead window while the application is |
| * crunching through the current window. |
| */ |
| if (ra->ahead_start == 0) { |
| ra->ahead_start = ra->start + ra->size; |
| ra->ahead_size = ra->next_size; |
| actual = do_page_cache_readahead(mapping, filp, |
| ra->ahead_start, ra->ahead_size); |
| check_ra_success(ra, ra->ahead_size, |
| actual, orig_next_size); |
| } |
| } |
| out: |
| return; |
| } |
| |
| /* |
| * For mmap reads (typically executables) the access pattern is fairly random, |
| * but somewhat ascending. So readaround favours pages beyond the target one. |
| * We also boost the window size, as it can easily shrink due to misses. |
| */ |
| void |
| page_cache_readaround(struct address_space *mapping, struct file_ra_state *ra, |
| struct file *filp, unsigned long offset) |
| { |
| if (ra->next_size != -1UL) { |
| const unsigned long min = get_min_readahead(ra) * 2; |
| unsigned long target; |
| unsigned long backward; |
| |
| /* |
| * If next_size is zero then leave it alone, because that's a |
| * readahead startup state. |
| */ |
| if (ra->next_size && ra->next_size < min) |
| ra->next_size = min; |
| |
| target = offset; |
| backward = ra->next_size / 4; |
| |
| if (backward > target) |
| target = 0; |
| else |
| target -= backward; |
| page_cache_readahead(mapping, ra, filp, target); |
| } |
| } |
| |
| /* |
| * handle_ra_miss() is called when it is known that a page which should have |
| * been present in the pagecache (we just did some readahead there) was in fact |
| * not found. This will happen if it was evicted by the VM (readahead |
| * thrashing) or if the readahead window is maximally shrunk. |
| * |
| * If the window has been maximally shrunk (next_size == 0) then bump it up |
| * again to resume readahead. |
| * |
| * Otherwise we're thrashing, so shrink the readahead window by three pages. |
| * This is because it is grown by two pages on a readahead hit. Theory being |
| * that the readahead window size will stabilise around the maximum level at |
| * which there is no thrashing. |
| */ |
| void handle_ra_miss(struct address_space *mapping, struct file_ra_state *ra) |
| { |
| const unsigned long min = get_min_readahead(ra); |
| |
| if (ra->next_size == -1UL) { |
| ra->next_size = min; |
| } else { |
| ra->next_size -= 3; |
| if (ra->next_size < min) |
| ra->next_size = min; |
| } |
| } |
| |
| /* |
| * Given a desired number of PAGE_CACHE_SIZE readahead pages, return a |
| * sensible upper limit. |
| */ |
| unsigned long max_sane_readahead(unsigned long nr) |
| { |
| unsigned long active; |
| unsigned long inactive; |
| |
| get_zone_counts(&active, &inactive); |
| return min(nr, inactive / 2); |
| } |