fs: track the inode having file locks with a flag in ->i_opflags

Opening and closing an inode dirties the ->i_readcount field.

Depending on the alignment of the inode, it may happen to false-share
with other fields loaded both for both operations to various extent.

This notably concerns the ->i_flctx field.

Since most inodes don't have the field populated, this bit can be managed
with a flag in ->i_opflags instead which bypasses the problem.

Here are results I obtained while opening a file read-only in a loop
with 24 cores doing the work on Sapphire Rapids. Utilizing the flag as
opposed to reading ->i_flctx field was toggled at runtime as the benchmark
was running, to make sure both results come from the same alignment.

before: 3233740
after:  3373346 (+4%)

before: 3284313
after:  3518711 (+7%)

before: 3505545
after:  4092806 (+16%)

Or to put it differently, this varies wildly depending on how (un)lucky
you get.

The primary bottleneck before and after is the avoidable lockref trip in
do_dentry_open().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251203094837.290654-2-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/locks.c

@@ -178,7 +178,6 @@ locks_get_lock_context(struct inode *inode, int type)
 {
 	struct file_lock_context *ctx;
 
-	/* paired with cmpxchg() below */
 	ctx = locks_inode_context(inode);
 	if (likely(ctx) || type == F_UNLCK)
 		goto out;
@@ -196,7 +195,18 @@ locks_get_lock_context(struct inode *inode, int type)
 	 * Assign the pointer if it's not already assigned. If it is, then
 	 * free the context we just allocated.
 	 */
-	if (cmpxchg(&inode->i_flctx, NULL, ctx)) {
+	spin_lock(&inode->i_lock);
+	if (!(inode->i_opflags & IOP_FLCTX)) {
+		VFS_BUG_ON_INODE(inode->i_flctx, inode);
+		WRITE_ONCE(inode->i_flctx, ctx);
+		/*
+		 * Paired with locks_inode_context().
+		 */
+		smp_store_release(&inode->i_opflags, inode->i_opflags | IOP_FLCTX);
+		spin_unlock(&inode->i_lock);
+	} else {
+		VFS_BUG_ON_INODE(!inode->i_flctx, inode);
+		spin_unlock(&inode->i_lock);
 		kmem_cache_free(flctx_cache, ctx);
 		ctx = locks_inode_context(inode);
 	}

include/linux/filelock.h

@@ -242,8 +242,12 @@ static inline struct file_lock_context *
 locks_inode_context(const struct inode *inode)
 {
 	/*
-	 * Paired with the fence in locks_get_lock_context().
+	 * Paired with smp_store_release in locks_get_lock_context().
+	 *
+	 * Ensures ->i_flctx will be visible if we spotted the flag.
 	 */
+	if (likely(!(smp_load_acquire(&inode->i_opflags) & IOP_FLCTX)))
+		return NULL;
 	return READ_ONCE(inode->i_flctx);
 }
 
@@ -471,7 +475,7 @@ static inline int break_lease(struct inode *inode, unsigned int mode)
 	 * could end up racing with tasks trying to set a new lease on this
 	 * file.
 	 */
-	flctx = READ_ONCE(inode->i_flctx);
+	flctx = locks_inode_context(inode);
 	if (!flctx)
 		return 0;
 	smp_mb();
@@ -490,7 +494,7 @@ static inline int break_deleg(struct inode *inode, unsigned int flags)
 	 * could end up racing with tasks trying to set a new lease on this
 	 * file.
 	 */
-	flctx = READ_ONCE(inode->i_flctx);
+	flctx = locks_inode_context(inode);
 	if (!flctx)
 		return 0;
 	smp_mb();
@@ -535,8 +539,11 @@ static inline int break_deleg_wait(struct delegated_inode *di)
 
 static inline int break_layout(struct inode *inode, bool wait)
 {
+	struct file_lock_context *flctx;
+
 	smp_mb();
-	if (inode->i_flctx && !list_empty_careful(&inode->i_flctx->flc_lease)) {
+	flctx = locks_inode_context(inode);
+	if (flctx && !list_empty_careful(&flctx->flc_lease)) {
 		unsigned int flags = LEASE_BREAK_LAYOUT;
 
 		if (!wait)

include/linux/fs.h

@@ -631,6 +631,7 @@ is_uncached_acl(struct posix_acl *acl)
 #define IOP_MGTIME		0x0020
 #define IOP_CACHED_LINK		0x0040
 #define IOP_FASTPERM_MAY_EXEC	0x0080
+#define IOP_FLCTX		0x0100
 
 /*
  * Inode state bits.  Protected by inode->i_lock