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

[Andy Lutomirski]

On Thu, Dec 13, 2012 at 11:27 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxx>
>> ---
>>
>> 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).

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.

--Andy
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