From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f177.google.com (mail-wi0-f177.google.com [209.85.212.177]) by kanga.kvack.org (Postfix) with ESMTP id B81BC6B006E for ; Wed, 19 Nov 2014 11:09:52 -0500 (EST) Received: by mail-wi0-f177.google.com with SMTP id l15so2414798wiw.16 for ; Wed, 19 Nov 2014 08:09:52 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id m1si156585wiy.80.2014.11.19.08.09.51 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 19 Nov 2014 08:09:51 -0800 (PST) Message-ID: <546CC0CD.40906@suse.cz> Date: Wed, 19 Nov 2014 17:09:49 +0100 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [PATCH] Repeated fork() causes SLAB to grow without bound References: <502D42E5.7090403@redhat.com> <20120818000312.GA4262@evergreen.ssec.wisc.edu> <502F100A.1080401@redhat.com> <20120822032057.GA30871@google.com> <50345232.4090002@redhat.com> <20130603195003.GA31275@evergreen.ssec.wisc.edu> <20141114163053.GA6547@cosmos.ssec.wisc.edu> <20141117160212.b86d031e1870601240b0131d@linux-foundation.org> <20141118014135.GA17252@cosmos.ssec.wisc.edu> <546AB1F5.6030306@redhat.com> <20141118121936.07b02545a0684b2cc839a10c@linux-foundation.org> <546BDB29.9050403@suse.cz> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Konstantin Khlebnikov Cc: Andrew Morton , Rik van Riel , Michel Lespinasse , Hugh Dickins , Andrea Arcangeli , Linux Kernel Mailing List , "linux-mm@kvack.org" , Tim Hartrick , Michal Hocko On 11/19/2014 03:36 PM, Konstantin Khlebnikov wrote: > On Wed, Nov 19, 2014 at 2:50 AM, Vlastimil Babka wrote: >> On 11/19/2014 12:02 AM, Konstantin Khlebnikov wrote: >>> On Wed, Nov 19, 2014 at 1:15 AM, Konstantin Khlebnikov wrote: >>>> On Tue, Nov 18, 2014 at 11:19 PM, Andrew Morton >>>> wrote: >>>>> On Mon, 17 Nov 2014 21:41:57 -0500 Rik van Riel wrote: >>>>> >>>>>> > Because of the serial forking there does indeed end up being an >>>>>> > infinite number of vmas. The initial vma can never be deleted >>>>>> > (even though the initial parent process has long since terminated) >>>>>> > because the initial vma is referenced by the children. >>>>>> >>>>>> There is a finite number of VMAs, but an infite number of >>>>>> anon_vmas. >>>>>> >>>>>> Subtle, yet deadly... >>>>> >>>>> Well, we clearly have the data structures screwed up. I've forgotten >>>>> enough about this code for me to be unable to work out what the fixed >>>>> up data structures would look like :( But surely there is some proper >>>>> solution here. Help? >>>> >>>> Not sure if it's right but probably we could reuse on fork an old anon_vma >>>> from the chain if it's already lost all vmas which points to it. >>>> For endlessly forking exploit this should work mostly like proposed patch >>>> which stops branching after some depth but without magic constant. >>> >>> Something like this. I leave proper comment for tomorrow. >> >> Hmm I'm not sure that will work as it is. If I understand it correctly, your >> patch can detect if the parent's anon_vma has no own references at the fork() >> time. But at the fork time, the parent is still alive, it only exits after the >> fork, right? So I guess it still has own references and the child will still >> allocate its new anon_vma, and the problem is not solved. > > But it could reuse anon_vma from grandparent or older. > Count of anon_vmas in chain will be limited with count of alive processes. Ah I missed that it can reuse older anon_vma, sorry. > I think it's better to describe this in terms of sets of anon_vma > instead hierarchy: > at clone vma inherits pages from parent together with set of anon_vma > which they belong. > For new pages it might allocate new anon_vma or reuse existing. After > my patch vma > will try to reuse anon_vma from that set which has no vmas which points to it. > As a result there will be no parent-child relation between anon_vma and > multiple pages might have equal (anon_vma, index) pair but I see no > problems here. Hmm I wonder if root anon_vma should be excluded from this reusal. For performance reasons, exclusive pages go to non-root anon_vma (see __page_set_anon_rmap()) and reusing root anon_vma would change this. Also from reading http://lwn.net/Articles/383162/ I understand that correctness also depends on the hierarchy and I wonder if there's a danger of reintroducing a bug like the one described there. Vlastimil >> >> So maybe we could detect that the own references dropped to zero when the parent >> does exit, and then change mapping of all relevant pages to the root anon_vma, >> destroy avc's of children and the anon_vma itself. But that sounds quite >> heavyweight :/ >> >> Vlastimil >> >>>> >>>>> >> > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org