linux-kernel - Re: [PATCH] thp+memcg-numa: fix BUG at include/linux/mm.h:370!

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Date:	Tue, 15 Mar 2011 08:42:57 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH] thp+memcg-numa: fix BUG at include/linux/mm.h:370!

On Mon, 14 Mar 2011 20:58:23 +0100
Johannes Weiner <jweiner@...hat.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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