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

[Al Viro]

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).
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