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>

2 files changed, 18 insertions, 11 deletions

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 93a0ba872ebe..69b0177da156 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -48,7 +48,7 @@ struct shmem_inode_info {
 	};
 	struct timespec64	i_crtime;	/* file creation time */
 	struct shared_policy	policy;		/* NUMA memory alloc policy */
-	struct simple_xattrs	*xattrs;	/* list of xattrs */
+	struct list_head        xattrs;		/* list of xattrs */
 	pgoff_t			fallocend;	/* highest fallocate endindex */
 	unsigned int		fsflags;	/* for FS_IOC_[SG]ETFLAGS */
 	atomic_t		stop_eviction;	/* hold when working on inode */
@@ -89,6 +89,7 @@ struct shmem_sb_info {
 	struct list_head shrinklist;  /* List of shinkable inodes */
 	unsigned long shrinklist_len; /* Length of shrinklist */
 	struct shmem_quota_limits qlimits; /* Default quota limits */
+	struct simple_xattr_cache xa_cache;
 };
 
 static inline struct shmem_inode_info *SHMEM_I(struct inode *inode)
diff --git a/include/linux/xattr.h b/include/linux/xattr.h
index ded446c1ef81..7aaaf4f8aff5 100644
--- a/include/linux/xattr.h
+++ b/include/linux/xattr.h
@@ -106,12 +106,14 @@ static inline const char *xattr_prefix(const struct xattr_handler *handler)
 	return handler->prefix ?: handler->name;
 }
 
-struct simple_xattrs {
-	struct rhashtable ht;
+struct simple_xattr_cache {
+	struct rhashtable *ht;
 };
 
 struct simple_xattr {
 	struct rhash_head hash_node;
+	struct list_head *parent;
+	struct list_head node;
 	struct rcu_head rcu;
 	char *name;
 	size_t size;
@@ -132,27 +134,31 @@ static inline void simple_xattr_limits_init(struct simple_xattr_limits *limits)
 	atomic_set(&limits->xattr_size, 0);
 }
 
-int simple_xattrs_init(struct simple_xattrs *xattrs);
-void simple_xattrs_free(struct simple_xattrs **xattrs, size_t *freed_space);
+void simple_xattrs_free(struct simple_xattr_cache *cache, struct list_head *xattrs,
+			size_t *freed_space);
 size_t simple_xattr_space(const char *name, size_t size);
 struct simple_xattr *simple_xattr_alloc(const void *value, size_t size);
 void simple_xattr_free(struct simple_xattr *xattr);
 void simple_xattr_free_rcu(struct simple_xattr *xattr);
-int simple_xattr_get(struct simple_xattrs **xattrs, const char *name,
-		     void *buffer, size_t size);
-struct simple_xattr *simple_xattr_set(struct simple_xattrs **xattrs,
+int simple_xattr_get(struct simple_xattr_cache *cache, struct list_head *xattrs,
+		     const char *name, void *buffer, size_t size);
+struct simple_xattr *simple_xattr_set(struct simple_xattr_cache *cache,
+				      struct list_head *xattrs,
 				      const char *name, const void *value,
 				      size_t size, int flags);
-int simple_xattr_set_limited(struct simple_xattrs **xattrs,
+int simple_xattr_set_limited(struct simple_xattr_cache *cache,
+			     struct list_head *xattrs,
 			     struct simple_xattr_limits *limits,
 			     const char *name, const void *value,
 			     size_t size, int flags);
-ssize_t simple_xattr_list(struct inode *inode, struct simple_xattrs **xattrs,
+ssize_t simple_xattr_list(struct inode *inode, struct list_head *xattrs,
 			  char *buffer, size_t size);
-int simple_xattr_add(struct simple_xattrs **xattrs,
+int simple_xattr_add(struct simple_xattr_cache *cache, struct list_head *xattrs,
 		     struct simple_xattr *new_xattr);
 int xattr_list_one(char **buffer, ssize_t *remaining_size, const char *name);
 
+void simple_xattr_cache_cleanup(struct simple_xattr_cache *cache);
+
 DEFINE_CLASS(simple_xattr,
 	     struct simple_xattr *,
 	     if (!IS_ERR_OR_NULL(_T)) simple_xattr_free(_T),