Re: [PATCH 3/4] fsnotify: count all objects with attached connectors

[Matthew Bobrowski <repnop@google.com>]

On Tue, Aug 03, 2021 at 09:03:43PM +0300, Amir Goldstein wrote:
> Rename s_fsnotify_inode_refs to s_fsnotify_conectors and count all

s/s_fsnotify_conectors/s_fsnotify_connectors ;)

> objects with attached connectors, not only inodes with attached
> connectors.
> 
> This will be used to optimize fsnotify() calls on sb without any
> type of marks.
> 
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Have one question below.

> ---
>  fs/notify/fsnotify.c |  6 +++---
>  fs/notify/mark.c     | 45 +++++++++++++++++++++++++++++++++++++++++---
>  include/linux/fs.h   |  4 ++--
>  3 files changed, 47 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
> index 30d422b8c0fc..a5de7f32c493 100644
> --- a/fs/notify/fsnotify.c
> +++ b/fs/notify/fsnotify.c
> @@ -87,9 +87,9 @@ static void fsnotify_unmount_inodes(struct super_block *sb)
>  
>  	if (iput_inode)
>  		iput(iput_inode);
> -	/* Wait for outstanding inode references from connectors */
> -	wait_var_event(&sb->s_fsnotify_inode_refs,
> -		       !atomic_long_read(&sb->s_fsnotify_inode_refs));
> +	/* Wait for outstanding object references from connectors */
> +	wait_var_event(&sb->s_fsnotify_connectors,
> +		       !atomic_long_read(&sb->s_fsnotify_connectors));
>  }
>  
>  void fsnotify_sb_delete(struct super_block *sb)
> diff --git a/fs/notify/mark.c b/fs/notify/mark.c
> index 2d8c46e1167d..622bcbface4f 100644
> --- a/fs/notify/mark.c
> +++ b/fs/notify/mark.c
> @@ -172,7 +172,7 @@ static void fsnotify_connector_destroy_workfn(struct work_struct *work)
>  static void fsnotify_get_inode_ref(struct inode *inode)
>  {
>  	ihold(inode);
> -	atomic_long_inc(&inode->i_sb->s_fsnotify_inode_refs);
> +	atomic_long_inc(&inode->i_sb->s_fsnotify_connectors);
>  }
>  
>  static void fsnotify_put_inode_ref(struct inode *inode)
> @@ -180,8 +180,45 @@ static void fsnotify_put_inode_ref(struct inode *inode)
>  	struct super_block *sb = inode->i_sb;
>  
>  	iput(inode);
> -	if (atomic_long_dec_and_test(&sb->s_fsnotify_inode_refs))
> -		wake_up_var(&sb->s_fsnotify_inode_refs);
> +	if (atomic_long_dec_and_test(&sb->s_fsnotify_connectors))
> +		wake_up_var(&sb->s_fsnotify_connectors);
> +}
> +
> +static void fsnotify_get_sb_connectors(struct fsnotify_mark_connector *conn)
> +{
> +	struct super_block *sb;
> +
> +	if (conn->type == FSNOTIFY_OBJ_TYPE_DETACHED)
> +		return;
> +
> +	if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
> +		sb = fsnotify_conn_inode(conn)->i_sb;
> +	else if (conn->type == FSNOTIFY_OBJ_TYPE_VFSMOUNT)
> +		sb = fsnotify_conn_mount(conn)->mnt.mnt_sb;
> +	else if (conn->type == FSNOTIFY_OBJ_TYPE_SB)
> +		sb = fsnotify_conn_sb(conn);

I noticed that you haven't provided an explicit case when no conditions are
matched, however this scenario appears to be handled in
fsnotify_put_sb_connectors() below. Why is this the case here and not in
fsnotify_put_sb_connectors()?

Also, I'm wondering if these blocks of code would be better expressed in a
switch statement. Alternatively, if these conditionals are shared across
the two helpers, why not factor out the super_block retrieval into an
inline helper just to simplify the callsite and not duplicate code? That's
of course if there is commonality between the two helpers.

> +
> +	atomic_long_inc(&sb->s_fsnotify_connectors);
> +}
> +
> +static void fsnotify_put_sb_connectors(struct fsnotify_mark_connector *conn)
> +{
> +	struct super_block *sb;
> +
> +	if (conn->type == FSNOTIFY_OBJ_TYPE_DETACHED)
> +		return;
> +
> +	if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
> +		sb = fsnotify_conn_inode(conn)->i_sb;
> +	else if (conn->type == FSNOTIFY_OBJ_TYPE_VFSMOUNT)
> +		sb = fsnotify_conn_mount(conn)->mnt.mnt_sb;
> +	else if (conn->type == FSNOTIFY_OBJ_TYPE_SB)
> +		sb = fsnotify_conn_sb(conn);
> +	else
> +		return;
> +
> +	if (atomic_long_dec_and_test(&sb->s_fsnotify_connectors))
> +		wake_up_var(&sb->s_fsnotify_connectors);
>  }
>  
>  static void *fsnotify_detach_connector_from_object(
> @@ -203,6 +240,7 @@ static void *fsnotify_detach_connector_from_object(
>  		fsnotify_conn_sb(conn)->s_fsnotify_mask = 0;
>  	}
>  
> +	fsnotify_put_sb_connectors(conn);
>  	rcu_assign_pointer(*(conn->obj), NULL);
>  	conn->obj = NULL;
>  	conn->type = FSNOTIFY_OBJ_TYPE_DETACHED;
> @@ -504,6 +542,7 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
>  		inode = fsnotify_conn_inode(conn);
>  		fsnotify_get_inode_ref(inode);
>  	}
> +	fsnotify_get_sb_connectors(conn);
>  
>  	/*
>  	 * cmpxchg() provides the barrier so that readers of *connp can see
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 640574294216..d48d2018dfa4 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1507,8 +1507,8 @@ struct super_block {
>  	/* Number of inodes with nlink == 0 but still referenced */
>  	atomic_long_t s_remove_count;
>  
> -	/* Pending fsnotify inode refs */
> -	atomic_long_t s_fsnotify_inode_refs;
> +	/* Number of inode/mount/sb objects that are being watched */
> +	atomic_long_t s_fsnotify_connectors;
>  
>  	/* Being remounted read-only */
>  	int s_readonly_remount;
> -- 
> 2.25.1

/M