Re: [PATCH v6 3/7] kernfs: use VFS negative dentry caching

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 9 Jun 2021 at 10:50, Ian Kent <raven@themaw.net> wrote:
>
> If there are many lookups for non-existent paths these negative lookups
> can lead to a lot of overhead during path walks.
>
> The VFS allows dentries to be created as negative and hashed, and caches
> them so they can be used to reduce the fairly high overhead alloc/free
> cycle that occurs during these lookups.
>
> Use the kernfs node parent revision to identify if a change has been
> made to the containing directory so that the negative dentry can be
> discarded and the lookup redone.
>
> Signed-off-by: Ian Kent <raven@themaw.net>
> ---
>  fs/kernfs/dir.c |   52 ++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 32 insertions(+), 20 deletions(-)
>
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index b3d1bc0f317d0..4f037456a8e17 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -1039,9 +1039,28 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
>         if (flags & LOOKUP_RCU)
>                 return -ECHILD;
>
> -       /* Always perform fresh lookup for negatives */
> -       if (d_really_is_negative(dentry))
> -               goto out_bad_unlocked;
> +       /* Negative hashed dentry? */
> +       if (d_really_is_negative(dentry)) {
> +               struct dentry *d_parent = dget_parent(dentry);
> +               struct kernfs_node *parent;
> +
> +               /* If the kernfs parent node has changed discard and
> +                * proceed to ->lookup.
> +                */
> +               parent = kernfs_dentry_node(d_parent);
> +               if (parent) {
> +                       if (kernfs_dir_changed(parent, dentry)) {

Perhaps add a note about this being dependent on parent of a negative
dentry never changing.

If this was backported to a kernel where this assumption doesn't hold,
there would be a mathematical chance of a false negative.

Thanks,
Miklos