linux-kernel - Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on mmap

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Date:	Fri, 14 Dec 2012 07:27:55 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Michel Lespinasse <walken@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, J??rn Engel <joern@...fs.org>
Subject: Re: [PATCH] mm: Downgrade mmap_sem before locking or populating on
 mmap

On Thu, Dec 13, 2012 at 09:49:43PM -0800, Andy Lutomirski wrote:
> This is a serious cause of mmap_sem contention.  MAP_POPULATE
> and MCL_FUTURE, in particular, are disastrous in multithreaded programs.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
> 
> Sensible people use anonymous mappings.  I write kernel patches :)
> 
> I'm not entirely thrilled by the aesthetics of this patch.  The MAP_POPULATE case
> could also be improved by doing it without any lock at all.  This is still a big
> improvement, though.

Wait a minute.  get_user_pages() relies on ->mmap_sem being held.  Unless
I'm seriously misreading your patch it removes that protection.  And yes,
I'm aware of execve-related exception; it's in special circumstances -
bprm->mm is guaranteed to be not shared (and we need to rearchitect that
area anyway, but that's a separate story).
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