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SubjectRe: [PATCH] thp+memcg-numa: fix BUG at include/linux/mm.h:370!
On Mon, 14 Mar 2011 20:58:23 +0100
Johannes Weiner <jweiner@redhat.com> wrote:

> On Mon, Mar 14, 2011 at 06:17:31PM +0100, Andrea Arcangeli wrote:
> > On Mon, Mar 14, 2011 at 09:56:10AM -0700, Linus Torvalds wrote:
> > > Does mem_cgroup_newpage_charge() even _need_ the mmap_sem at all? And
> > > if not, why not release the read-lock early? And even if it _does_
> > > need it, why not do
>
> [...]
>
> > About mem_cgroup_newpage_charge I think you're right it won't need the
> > mmap_sem. Running it under it is sure safe. But if it's not needed we
> > can move the up_read before the mem_cgroup_newpage_charge like you
> > suggested. Johannes/Minchan could you confirm the mmap_sem isn't
> > needed around mem_cgroup_newpage_charge? The mm and new_page are
> > stable without the mmap_sem, only the vma goes away but the memcg
> > shouldn't care.
>
> We don't care about the vma. It's all about assigning the physical
> page to the memcg that mm->owner belongs to.
>
> It would be the first callsite not holding the mmap_sem, but that is
> only because all existing sites are fault handlers that don't drop the
> lock for other reasons.
>
> I am not aware of anything that would rely on the lock in there, or
> would not deserve to break if it did.
>

mmap_sem is not required to held if uncharge() operation is done
if vma turns out to be a stale pointer.

Thanks,
-Kame



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