linux-kernel - Re: [RFC PATCH -v3] take all anon_vma locks in anon_vma

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4BD8EA85.2000209@redhat.com>
Date:	Wed, 28 Apr 2010 22:10:13 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Minchan Kim <minchan.kim@...il.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH -v3] take all anon_vma locks in anon_vma_lock

On 04/28/2010 08:28 PM, Minchan Kim wrote:
> On Thu, Apr 29, 2010 at 5:57 AM, Rik van Riel<riel@...hat.com>  wrote:
>> Take all the locks for all the anon_vmas in anon_vma_lock, this properly
>> excludes migration and the transparent hugepage code from VMA changes done
>> by mmap/munmap/mprotect/expand_stack/etc...
>>
>> Unfortunately, this requires adding a new lock (mm->anon_vma_chain_lock),
>> otherwise we have an unavoidable lock ordering conflict.  This changes the
>> locking rules for the "same_vma" list to be either mm->mmap_sem for write,
>> or mm->mmap_sem for read plus the new mm->anon_vma_chain lock.  This limits
>> the place where the new lock is taken to 2 locations - anon_vma_prepare and
>> expand_downwards.
>>
>> Document the locking rules for the same_vma list in the anon_vma_chain and
>> remove the anon_vma_lock call from expand_upwards, which does not need it.
>>
>> Signed-off-by: Rik van Riel<riel@...hat.com>
>
> This patch makes things simple. So I like this.
> Actually, I wanted this all-at-once locks approach.
> But I was worried about that how the patch affects AIM 7 workload
> which is cause of anon_vma_chain about scalability by Rik.
> But now Rik himself is sending the patch. So I assume the patch
> couldn't decrease scalability of the workload heavily.

The thing is, the number of anon_vmas attached to a VMA is
small (depth of the tree, so for apache or aim the typical
depth is 2). This N is between 1 and 3.

The problem we had originally is the _width_ of the tree,
where every sibling process was attached to the same anon_vma
and the rmap code had to walk the page tables of all the
processes, for every privately owned page in each child process.
For large server workloads, this N is between a few hundred and
a few thousand.

What matters most at this point is correctness - we need to be
able to exclude rmap walks when messing with a VMA in any way
that breaks lookups, because rmap walks for page migration and
hugepage conversion have to be 100% reliable.

That is not a constraint I had in mind with the original
anon_vma changes, so the code needs to be fixed up now...

I suspect that taking one or two extra spinlocks in the code
paths changed by this patch (mmap/munmap/...) is going to make
a difference at all, since all of those paths are pretty
infrequently taken.

-- 
All rights reversed
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/