linux-kernel - Re: [RFC] vfs: don't bother clearing close_on

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Date:	Wed, 04 Nov 2015 04:33:28 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] vfs: don't bother clearing close_on_exec bit for unused
 fds

On Wed, 2015-11-04 at 11:59 +0100, Rasmus Villemoes wrote:

> @@ -667,10 +665,9 @@ void do_close_on_exec(struct files_struct *files)
>  		fdt = files_fdtable(files);
>  		if (fd >= fdt->max_fds)
>  			break;
> -		set = fdt->close_on_exec[i];
> +		set = fdt->close_on_exec[i] & fdt->open_fds[i];
>  		if (!set)
>  			continue;


Many processes have a big hole at the end of fdt->open_fds[], due
to the fact that max_fds is rounded to a power of two.

It makes sense to avoid bringing in cpu caches the close_on_exec[] part.


set = fdt->open_fds[i];
if (!set)
	continue;
set &= fdt->close_on_exec[i];
if (!set)
	continue;

Not sure if this is a net win due to branch prediction...


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