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

[Ian Kent <raven@themaw.net>]

On Fri, 2021-06-11 at 15:07 +0200, Miklos Szeredi wrote:
> 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.

Which of course it it can change, at any time.

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

Isn't this a cunning way of saying "in thinking about the move case
you've forgotten about the obvious common case, just put back taking
the read lock already, at least for the check"?

Ian