linux-kernel - Re: [PATCH] fs: mark lookup

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Message-ID: <20251125-punkten-jegliche-5aee8187381d@brauner>
Date: Tue, 25 Nov 2025 10:05:50 +0100
From: Christian Brauner <brauner@...nel.org>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: viro@...iv.linux.org.uk, jack@...e.cz, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: mark lookup_slow() as noinline

On Wed, Nov 19, 2025 at 03:49:30PM +0100, Mateusz Guzik wrote:
> Otherwise it gets inlined notably in walk_component(), which convinces
> the compiler to push/pop additional registers in the fast path to
> accomodate existence of the inlined version.
> 
> Shortens the fast path of that routine from 87 to 71 bytes.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> ---

Fwiw, the biggest problem is that we need to end up with something
obvious so that we don't accidently destroy any potential performance
gain in say 2 years because everyone forgot why we did things this way.

> 
> The intent is to get to a point where I can inline walk_component and
> step_into. This is probably the last patch of the sort before I write a
> patchset + provide bench results.
> 
>  fs/namei.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 11295fcf877c..667360deef48 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1863,7 +1863,7 @@ static struct dentry *__lookup_slow(const struct qstr *name,
>  	return dentry;
>  }
>  
> -static struct dentry *lookup_slow(const struct qstr *name,
> +static noinline struct dentry *lookup_slow(const struct qstr *name,
>  				  struct dentry *dir,
>  				  unsigned int flags)
>  {
> -- 
> 2.48.1
>