On Wed, 9 Jun 2021 at 10:50, Ian Kent <raven@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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
