Re: [RFC PATCH] mm/migration: Remove anon vma locking fromtry_to_unmap() use

[Linus Torvalds]

On Sat, Dec 1, 2012 at 4:26 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> So as a quick concept hack I wrote the patch attached below.
> (It's not signed off, see the patch description text for the
> reason.)

Well, it confirms that anon_vma locking is a big problem, but as
outlined in my other email it's completely incorrect from an actual
behavior standpoint.

Btw, I think the anon_vma lock could be made a spinlock instead of a
mutex or rwsem, but that would probably take more work. We *shouldn't*
be doing anything that needs IO inside the anon_vma lock, though, so
it *should* be doable. But there are probably quite a bit of
allocations inside the lock, and I know it covers huge areas, so a
spinlock might not only be hard to convert to, it quite likely has
latency issues too.

Oh, btw, MUTEX_SPIN_ON_OWNER may well improve performance too, but it
gets disabled by DEBUG_MUTEXES. So some of the performance impact of
the vma locking may be *very* kernel-config dependent.

                  Linus
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