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

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Message-ID: <3xwv7kza6hgxfzzsmyoolno4yygiqses4rutu3n2l2qqrf56ry@p7hs7s5yik2t>
Date: Thu, 27 Nov 2025 14:52:50 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	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 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:

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

								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