simpe_xattr: use per-sb cache

Move the hash table to the super block to remove excessive overhead in case
of small number of xattrs per inode.

Add linked list to the inode, used for listxattr and eviction.  Listxattr
uses rcu protection to iterate the list of xattrs.

Before being made per-sb, lazy allocation was protected by inode lock.  Now
inode lock no longer provides sufficient exclusion, so use cmpxchg() to
ensure atomicity.

Though I haven't found a description of this pattern, after some research
it seems that cmpxchg_release() and READ_ONCE() should provide the
necessary memory barriers.

Use simple_xattr_free_rcu() in simple_xattrs_free(). This is needed because
the hash table is now shared between inodes and lookup on a different inode
might be running the compare function on the just freed element within the
RCU grace period.

Following stats are based on slabinfo diff, after creating 100k empty
files, then adding a "user.test=foo" xattr to each:

v7.0 (no rhashtable):
  File creation: 993.40 bytes/file
  Xattr addition: 79.99 bytes/file

v7.1-rc2 (per-inode rhashtable):
  File creation: 939.73 bytes/file
  Xattr addition: 1296.08 bytes/file

v7.1-rc2 + this patch (per-sb rhashtable)
  File creation: 946.84 bytes/file
  Xattr addition: 111.86 bytes/file

The overhead of a single xattr is reduced to nearly v7.0 levels.  The per
xattr overhead is slightly larger due to the addition of three pointers to
struct simple_xattr.

Fixes: b32c4a213698 ("xattr: add rhashtable-based simple_xattr infrastructure")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://patch.msgid.link/20260605135322.2632068-5-mszeredi@redhat.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index cf4ee9f41191..7b1ea9fb598f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1425,7 +1425,7 @@ static void shmem_evict_inode(struct inode *inode)
 		}
 	}
 
-	simple_xattrs_free(&info->xattrs, sbinfo->max_inodes ? &freed : NULL);
+	simple_xattrs_free(&sbinfo->xa_cache, &info->xattrs, sbinfo->max_inodes ? &freed : NULL);
 
 	shmem_free_inode(inode->i_sb, freed);
 	WARN_ON(inode->i_blocks);
@@ -3084,6 +3084,7 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
 	inode->i_generation = get_random_u32();
 	info = SHMEM_I(inode);
 	memset(info, 0, (char *)inode - (char *)info);
+	INIT_LIST_HEAD_RCU(&info->xattrs);
 	spin_lock_init(&info->lock);
 	atomic_set(&info->stop_eviction, 0);
 	info->seals = F_SEAL_SEAL;
@@ -4258,7 +4259,7 @@ static int shmem_initxattrs(struct inode *inode,
 		if (!new_xattr->name)
 			break;
 
-		if (simple_xattr_add(&info->xattrs, new_xattr))
+		if (simple_xattr_add(&sbinfo->xa_cache, &info->xattrs, new_xattr))
 			break;
 
 		if (sbinfo->max_inodes)
@@ -4283,10 +4284,11 @@ static int shmem_xattr_handler_get(const struct xattr_handler *handler,
 				   struct dentry *unused, struct inode *inode,
 				   const char *name, void *buffer, size_t size)
 {
+	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	struct shmem_inode_info *info = SHMEM_I(inode);
 
 	name = xattr_full_name(handler, name);
-	return simple_xattr_get(&info->xattrs, name, buffer, size);
+	return simple_xattr_get(&sbinfo->xa_cache, &info->xattrs, name, buffer, size);
 }
 
 static int shmem_xattr_handler_set(const struct xattr_handler *handler,
@@ -4314,7 +4316,7 @@ static int shmem_xattr_handler_set(const struct xattr_handler *handler,
 			return -ENOSPC;
 	}
 
-	old_xattr = simple_xattr_set(&info->xattrs, name, value, size, flags);
+	old_xattr = simple_xattr_set(&sbinfo->xa_cache, &info->xattrs, name, value, size, flags);
 	if (!IS_ERR(old_xattr)) {
 		ispace = 0;
 		if (old_xattr && sbinfo->max_inodes)
@@ -4963,6 +4965,9 @@ static void shmem_put_super(struct super_block *sb)
 	free_percpu(sbinfo->ino_batch);
 	percpu_counter_destroy(&sbinfo->used_blocks);
 	mpol_put(sbinfo->mpol);
+#ifdef CONFIG_TMPFS_XATTR
+	simple_xattr_cache_cleanup(&sbinfo->xa_cache);
+#endif
 	kfree(sbinfo);
 	sb->s_fs_info = NULL;
 }