Re: [PATCH v2 2/2] fs: track the inode having file locks with a flag in ->i_opflags

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2025-11-12 at 11:44 +0100, Mateusz Guzik wrote:
> 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().
> 
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
> ---
> 
> v2:
> - fix smatch warning:
> ./include/linux/filelock.h:241:24: warning: Using plain integer as NULL pointer
> 
>  fs/locks.c               | 14 ++++++++++++--
>  include/linux/filelock.h | 15 +++++++++++----
>  include/linux/fs.h       |  1 +
>  3 files changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index 04a3f0e20724..9cd23dd2a74e 100644
> --- a/fs/locks.c
> +++ b/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);

Not thrilled with having to deal with the i_lock here, but this is the
slow path and we typically only go through it once per inode. I can
believe that IOP_FLCTX helps the more common case where the i_flctx is
already present.

> +	} 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);
>  	}
> diff --git a/include/linux/filelock.h b/include/linux/filelock.h
> index 37e1b33bd267..9dc57b3dcfae 100644
> --- a/include/linux/filelock.h
> +++ b/include/linux/filelock.h
> @@ -233,8 +233,12 @@ 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);
>  }
>  
> @@ -441,7 +445,7 @@ 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();
> @@ -460,7 +464,7 @@ static inline int break_deleg(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();
> @@ -493,8 +497,11 @@ static inline int break_deleg_wait(struct inode **delegated_inode)
>  
>  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))
>  		return __break_lease(inode,
>  				wait ? O_WRONLY : O_WRONLY | O_NONBLOCK,
>  				FL_LAYOUT);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index b803a592c07e..b3d9230c310f 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -655,6 +655,7 @@ 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

This looks correct to me.

Reviewed-by: Jeff Layton <jlayton@kernel.org>