On Mon, Jun 15, 2020 at 3:14 PM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Changelog since v1
> o Updated changelog
Slipped to commit message
>
> The kernel uses internal mounts created by kern_mount() and populated
> with files with no lookup path by alloc_file_pseudo for a variety of
> reasons. An example of such a mount is for anonymous pipes. For pipes,
> every vfs_write regardless of filesystem, fsnotify_modify() is called to
> notify of any changes which incurs a small amount of overhead in fsnotify
> even when there are no watchers. It can also trigger for reads and readv
> and writev, it was simply vfs_write() that was noticed first.
>
> A patch is pending that reduces, but does not eliminte, the overhead of
typo: eliminte
> fsnotify but for files that cannot be looked up via a path, even that
> small overhead is unnecessary. The user API for fanotify is based on
> the pathname and a dirfd and proc entries appear to be the only visible
> representation of the files. Proc does not have the same pathname as the
> internal entry and the proc inode is not the same as the internal inode
> so even if fanotify is used on a file under /proc/XX/fd, no useful events
> are notified.
>
Note that fanotify is not the only uapi to add marks, but this is fine by me
I suppose if Jan wants to he can make small corrections on commit.
> This patch changes alloc_file_pseudo() to always opt out of fsnotify by
> setting FMODE_NONOTIFY flag so that no check is made for fsnotify watchers
> on pseudo files. This should be safe as the underlying helper for the
> dentry is d_alloc_pseudo which explicitly states that no lookups are ever
> performed meaning that fanotify should have nothing useful to attach to.
>
> The test motivating this was "perf bench sched messaging --pipe". On
> a single-socket machine using threads the difference of the patch was
> as follows.
>
> 5.7.0 5.7.0
> vanilla nofsnotify-v1r1
> Amean 1 1.3837 ( 0.00%) 1.3547 ( 2.10%)
> Amean 3 3.7360 ( 0.00%) 3.6543 ( 2.19%)
> Amean 5 5.8130 ( 0.00%) 5.7233 * 1.54%*
> Amean 7 8.1490 ( 0.00%) 7.9730 * 2.16%*
> Amean 12 14.6843 ( 0.00%) 14.1820 ( 3.42%)
> Amean 18 21.8840 ( 0.00%) 21.7460 ( 0.63%)
> Amean 24 28.8697 ( 0.00%) 29.1680 ( -1.03%)
> Amean 30 36.0787 ( 0.00%) 35.2640 * 2.26%*
> Amean 32 38.0527 ( 0.00%) 38.1223 ( -0.18%)
>
> The difference is small but in some cases it's outside the noise so
> while marginal, there is still some small benefit to ignoring fsnotify
> for files allocated via alloc_file_pseudo in some cases.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Thanks,
Amir.
> ---
> fs/file_table.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 30d55c9a1744..0076ccf67a7d 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -229,7 +229,7 @@ struct file *alloc_file_pseudo(struct inode *inode, struct vfsmount *mnt,
> d_set_d_op(path.dentry, &anon_ops);
> path.mnt = mntget(mnt);
> d_instantiate(path.dentry, inode);
> - file = alloc_file(&path, flags, fops);
> + file = alloc_file(&path, flags | FMODE_NONOTIFY, fops);
> if (IS_ERR(file)) {
> ihold(inode);
> path_put(&path);
