linux-kernel - Re: [PATCH] rmap: make anon_vma_prepare link in all the anon

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Date:	Wed, 07 Apr 2010 10:48:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com, hannes@...xchg.org
Subject: Re: [PATCH] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA

On 04/07/2010 03:00 AM, KOSAKI Motohiro wrote:

> Hmm.. probably I'm moron.

Someone might be, but it's not you :)

> I'm also confusing this locking rule as same as linus said.
>
> after this patch, new locking order are

> So, Why mmap_sem read lock can protect vma->anon_vma_chain?
> An another threads seems to be able to change avc list concurrentlly and freely.

You are right, the code needs to take the pagetable_lock
around the call to anon_vma_clone, so other threads
get locked out.

This means the locking order has now been inverted,
with the pagetable_lock on the outside and the
anon_vma locks on the inside.

I have checked all the other call sites to the
anon_vma code.  The direct callers of anon_vma_clone
and anon_vma_fork already hold the mmap_sem for
write.  The callers of anon_vma_prepare hold the
mmap_sem for read - so excluding other callers of
anon_vma_prepare with the page_table_lock is enough.

mm_take_all_locks has the mmap_sem for write.

There seem to be no other traversals of the same_vma
list, so changing the locking order to have the
page_table_lock on the outside of the anon_vma locks
works.

> plus, Why don't we need "vma->anon_vma = merge_vma->anon_vma" assignment?
> if vma->anon_vma keep NULL, I think anon_vma_prepare() call anon_vma_clone()
> multiple times.

Added in the new version.  See the next email.

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