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Message-ID: <4BBC9B3A.3060703@redhat.com>
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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