linux-kernel - Re: [PATCH] dcache: predict the name matches if parent and length also match

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Message-ID: <CAGudoHGh-=MKK0CBWosD6ikb5HHmRr0u_YxQD0bceL9dCeUYDw@mail.gmail.com>
Date: Thu, 27 Nov 2025 14:55:09 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] dcache: predict the name matches if parent and length
 also match

On Thu, Nov 27, 2025 at 2:52 PM Jan Kara <jack@...e.cz> wrote:
>
> On Thu 27-11-25 13:24:12, Mateusz Guzik wrote:
> > dentry_cmp() has predicts inside, but they were not enough to convince
> > the compiler.
> >
> > As for difference in asm, some of the code is reshuffled and there is
> > one less unconditional jump to get there.
> >
> > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
>
> I've checked and on my laptop the dentry hash table has ~2 million entries.
> This means we'll be getting hash collisions within a directory for
> directories on the order of thousands entries. And until we get to hundreds
> of thousands of entries in a directory, the collisions of entries will be
> still rare. So I guess that's rare enough. Feel free to add:
>

collisions with the same parent are a given, but with the same length
on top should be rare

> Reviewed-by: Jan Kara <jack@...e.cz>

thanks, but see v2 :)
https://lore.kernel.org/linux-fsdevel/20251127131526.4137768-1-mjguzik@gmail.com/T/#u
>
>                                                                 Honza
>
> > ---
> >
> > i know it's late, but given the non-semantic-modifying nature of the
> > change, i think it can still make it for 6.19
> >
> >  fs/dcache.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 23d1752c29e6..bc84f89156fa 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -2346,7 +2346,7 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
> >                       continue;
> >               if (dentry->d_name.hash_len != hashlen)
> >                       continue;
> > -             if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
> > +             if (unlikely(dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0))
> >                       continue;
> >               *seqp = seq;
> >               return dentry;
> > --
> > 2.34.1
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR