On Thu, Dec 13, 2012 at 11:27 PM, Al Viro [off-list ref] wrote:

On Thu, Dec 13, 2012 at 09:49:43PM -0800, Andy Lutomirski wrote:

quoted

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@amacapital.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).

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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