patches/old/shmem-stable-directory-offsets.patch

From: Chuck Lever <chuck.lever@oracle.com>
Subject: shmem: stable directory offsets
Date: Fri, 30 Jun 2023 13:49:03 -0400

The current cursor-based directory offset mechanism doesn't work when a
tmpfs filesystem is exported via NFS.  This is because NFS clients do not
open directories.  Each server-side READDIR operation has to open the
directory, read it, then close it.  The cursor state for that directory,
being associated strictly with the opened struct file, is thus discarded
after each NFS READDIR operation.

Directory offsets are cached not only by NFS clients, but also by user
space libraries on those clients.  Essentially there is no way to
invalidate those caches when directory offsets have changed on an NFS
server after the offset-to-dentry mapping changes.  Thus the whole
application stack depends on unchanging directory offsets.

The solution we've come up with is to make the directory offset for each
file in a tmpfs filesystem stable for the life of the directory entry it
represents.

shmem_readdir() and shmem_dir_llseek() now use an xarray to map each
directory offset (an loff_t integer) to the memory address of a struct
dentry.

Link: https://lkml.kernel.org/r/168814734331.530310.3911190551060453102.stgit@manet.1015granger.net
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/shmem_fs.h |    1 
 mm/shmem.c               |   47 +++++++++++++++++++++++++++++++------
 2 files changed, 41 insertions(+), 7 deletions(-)

--- a/include/linux/shmem_fs.h~shmem-stable-directory-offsets
+++ a/include/linux/shmem_fs.h
@@ -27,6 +27,7 @@ struct shmem_inode_info {
 	atomic_t		stop_eviction;	/* hold when working on inode */
 	struct timespec64	i_crtime;	/* file creation time */
 	unsigned int		fsflags;	/* flags for FS_IOC_[SG]ETFLAGS */
+	struct offset_ctx	dir_offsets;	/* stable entry offsets */
 	struct inode		vfs_inode;
 };
 
--- a/mm/shmem.c~shmem-stable-directory-offsets
+++ a/mm/shmem.c
@@ -2372,6 +2372,11 @@ static void shmem_set_inode_flags(struct
 #define shmem_initxattrs NULL
 #endif
 
+static struct offset_ctx *shmem_get_offset_ctx(struct inode *inode)
+{
+	return &SHMEM_I(inode)->dir_offsets;
+}
+
 static struct inode *shmem_get_inode(struct mnt_idmap *idmap, struct super_block *sb,
 				     struct inode *dir, umode_t mode, dev_t dev,
 				     unsigned long flags)
@@ -2427,7 +2432,8 @@ static struct inode *shmem_get_inode(str
 			/* Some things misbehave if size == 0 on a directory */
 			inode->i_size = 2 * BOGO_DIRENT_SIZE;
 			inode->i_op = &shmem_dir_inode_operations;
-			inode->i_fop = &simple_dir_operations;
+			inode->i_fop = &simple_offset_dir_operations;
+			simple_offset_init(shmem_get_offset_ctx(inode));
 			break;
 		case S_IFLNK:
 			/*
@@ -3099,7 +3105,10 @@ shmem_mknod(struct mnt_idmap *idmap, str
 		if (error && error != -EOPNOTSUPP)
 			goto out_iput;
 
-		error = 0;
+		error = simple_offset_add(shmem_get_offset_ctx(dir), dentry);
+		if (error)
+			goto out_iput;
+
 		dir->i_size += BOGO_DIRENT_SIZE;
 		dir->i_ctime = dir->i_mtime = current_time(dir);
 		inode_inc_iversion(dir);
@@ -3176,6 +3185,13 @@ static int shmem_link(struct dentry *old
 			goto out;
 	}
 
+	ret = simple_offset_add(shmem_get_offset_ctx(dir), dentry);
+	if (ret) {
+		if (inode->i_nlink)
+			shmem_free_inode(inode->i_sb);
+		goto out;
+	}
+
 	dir->i_size += BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
@@ -3194,6 +3210,8 @@ static int shmem_unlink(struct inode *di
 	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
 		shmem_free_inode(inode->i_sb);
 
+	simple_offset_remove(shmem_get_offset_ctx(dir), dentry);
+
 	dir->i_size -= BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
@@ -3252,24 +3270,29 @@ static int shmem_rename2(struct mnt_idma
 {
 	struct inode *inode = d_inode(old_dentry);
 	int they_are_dirs = S_ISDIR(inode->i_mode);
+	int error;
 
 	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))
 		return -EINVAL;
 
 	if (flags & RENAME_EXCHANGE)
-		return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
+		return simple_offset_rename_exchange(old_dir, old_dentry,
+						     new_dir, new_dentry);
 
 	if (!simple_empty(new_dentry))
 		return -ENOTEMPTY;
 
 	if (flags & RENAME_WHITEOUT) {
-		int error;
-
 		error = shmem_whiteout(idmap, old_dir, old_dentry);
 		if (error)
 			return error;
 	}
 
+	simple_offset_remove(shmem_get_offset_ctx(old_dir), old_dentry);
+	error = simple_offset_add(shmem_get_offset_ctx(new_dir), old_dentry);
+	if (error)
+		return error;
+
 	if (d_really_is_positive(new_dentry)) {
 		(void) shmem_unlink(new_dir, new_dentry);
 		if (they_are_dirs) {
@@ -3313,19 +3336,23 @@ static int shmem_symlink(struct mnt_idma
 	if (error && error != -EOPNOTSUPP)
 		goto out_iput;
 
+	error = simple_offset_add(shmem_get_offset_ctx(dir), dentry);
+	if (error)
+		goto out_iput;
+
 	inode->i_size = len-1;
 	if (len <= SHORT_SYMLINK_LEN) {
 		inode->i_link = kmemdup(symname, len, GFP_KERNEL);
 		if (!inode->i_link) {
 			error = -ENOMEM;
-			goto out_iput;
+			goto out_remove_offset;
 		}
 		inode->i_op = &shmem_short_symlink_operations;
 	} else {
 		inode_nohighmem(inode);
 		error = shmem_get_folio(inode, 0, &folio, SGP_WRITE);
 		if (error)
-			goto out_iput;
+			goto out_remove_offset;
 		inode->i_mapping->a_ops = &shmem_aops;
 		inode->i_op = &shmem_symlink_inode_operations;
 		memcpy(folio_address(folio), symname, len);
@@ -3340,6 +3367,9 @@ static int shmem_symlink(struct mnt_idma
 	d_instantiate(dentry, inode);
 	dget(dentry);
 	return 0;
+
+out_remove_offset:
+	simple_offset_remove(shmem_get_offset_ctx(dir), dentry);
 out_iput:
 	iput(inode);
 	return error;
@@ -4072,6 +4102,8 @@ static void shmem_destroy_inode(struct i
 {
 	if (S_ISREG(inode->i_mode))
 		mpol_free_shared_policy(&SHMEM_I(inode)->policy);
+	if (S_ISDIR(inode->i_mode))
+		simple_offset_destroy(shmem_get_offset_ctx(inode));
 }
 
 static void shmem_init_inode(void *foo)
@@ -4152,6 +4184,7 @@ static const struct inode_operations shm
 	.mknod		= shmem_mknod,
 	.rename		= shmem_rename2,
 	.tmpfile	= shmem_tmpfile,
+	.get_offset_ctx	= shmem_get_offset_ctx,
 #endif
 #ifdef CONFIG_TMPFS_XATTR
 	.listxattr	= shmem_listxattr,
_