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

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Date:	Fri, 14 Dec 2012 08:12:45 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
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 Fri, Dec 14, 2012 at 6:49 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Fri, Dec 14, 2012 at 03:14:50AM -0800, Andy Lutomirski wrote:
>
>> > 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).
>>
>> Unless I completely screwed up the patch, ->mmap_sem is still held for
>> read (it's downgraded from write).  It's just not held for write
>> anymore.
>
> Huh?  I'm talking about the call of get_user_pages() in aio_setup_ring().
> With your patch it's done completely outside of ->mmap_sem, isn't it?

Oh, /that/ call to get_user_pages.  That would qualify as screwing up...

Since dropping and reacquiring mmap_sem there is probably a bad idea
there, I'll rework this and post a v2.

--Andy
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