Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751700AbaFDIxf (ORCPT ); Wed, 4 Jun 2014 04:53:35 -0400 Received: from mail-vc0-f176.google.com ([209.85.220.176]:52605 "EHLO mail-vc0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751664AbaFDIxa (ORCPT ); Wed, 4 Jun 2014 04:53:30 -0400 MIME-Version: 1.0 In-Reply-To: <20140603081655.GA6013@esperanza> References: <5d2fbc894a2c62597e7196bb1ebb8357b15529ab.1401457502.git.vdavydov@parallels.com> <20140531110456.GC25076@esperanza> <20140602042435.GA17964@js1304-P5Q-DELUXE> <20140602114741.GA1039@esperanza> <20140603081655.GA6013@esperanza> Date: Wed, 4 Jun 2014 17:53:29 +0900 Message-ID: Subject: Re: [PATCH -mm 7/8] slub: make dead caches discard free slabs immediately From: Joonsoo Kim To: Vladimir Davydov Cc: Joonsoo Kim , Christoph Lameter , Andrew Morton , Johannes Weiner , Michal Hocko , LKML , Linux Memory Management List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2014-06-03 17:16 GMT+09:00 Vladimir Davydov : > On Mon, Jun 02, 2014 at 11:03:51PM +0900, Joonsoo Kim wrote: >> 2014-06-02 20:47 GMT+09:00 Vladimir Davydov : >> > Hi Joonsoo, >> > >> > On Mon, Jun 02, 2014 at 01:24:36PM +0900, Joonsoo Kim wrote: >> >> On Sat, May 31, 2014 at 03:04:58PM +0400, Vladimir Davydov wrote: >> >> > On Fri, May 30, 2014 at 09:57:10AM -0500, Christoph Lameter wrote: >> >> > > On Fri, 30 May 2014, Vladimir Davydov wrote: >> >> > > >> >> > > > (3) is a bit more difficult, because slabs are added to per-cpu partial >> >> > > > lists lock-less. Fortunately, we only have to handle the __slab_free >> >> > > > case, because, as there shouldn't be any allocation requests dispatched >> >> > > > to a dead memcg cache, get_partial_node() should never be called. In >> >> > > > __slab_free we use cmpxchg to modify kmem_cache_cpu->partial (see >> >> > > > put_cpu_partial) so that setting ->partial to a special value, which >> >> > > > will make put_cpu_partial bail out, will do the trick. >> > [...] >> >> I think that we can do (3) easily. >> >> If we check memcg_cache_dead() in the end of put_cpu_partial() rather >> >> than in the begin of put_cpu_partial(), we can avoid the race you >> >> mentioned. If someone do put_cpu_partial() before dead flag is set, >> >> it can be zapped by who set dead flag. And if someone do >> >> put_cpu_partial() after dead flag is set, it can be zapped by who >> >> do put_cpu_partial(). >> > >> > After put_cpu_partial() adds a frozen slab to a per cpu partial list, >> > the slab becomes visible to other threads, which means it can be >> > unfrozen and freed. The latter can trigger cache destruction. Hence we >> > shouldn't touch the cache, in particular call memcg_cache_dead() on it, >> > after calling put_cpu_partial(), otherwise we can get use-after-free. >> > >> > However, what you propose makes sense if we disable irqs before adding a >> > slab to a partial list and enable them only after checking if the cache >> > is dead and unfreezing all partials if so, i.e. >> > >> > diff --git a/mm/slub.c b/mm/slub.c >> > index d96faa2464c3..14b9e9a8677c 100644 >> > --- a/mm/slub.c >> > +++ b/mm/slub.c >> > @@ -2030,8 +2030,15 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain) >> > struct page *oldpage; >> > int pages; >> > int pobjects; >> > + unsigned long flags; >> > + int irq_saved = 0; >> > >> > do { >> > + if (irq_saved) { >> > + local_irq_restore(flags); >> > + irq_saved = 0; >> > + } >> > + >> > pages = 0; >> > pobjects = 0; >> > oldpage = this_cpu_read(s->cpu_slab->partial); >> > @@ -2062,8 +2069,16 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain) >> > page->pobjects = pobjects; >> > page->next = oldpage; >> > >> > + local_irq_save(flags); >> > + irq_saved = 1; >> > + >> > } while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page) >> > != oldpage); >> > + >> > + if (memcg_cache_dead(s)) >> > + unfreeze_partials(s, this_cpu_ptr(s->cpu_slab)); >> > + >> > + local_irq_restore(flags); >> > #endif >> > } >> > >> > >> > That would be safe against possible cache destruction, because to remove >> > a slab from a per cpu partial list we have to run on the cpu it was >> > frozen on. Disabling irqs makes it impossible. >> >> Hmm... this is also a bit ugly. >> How about following change? >> >> Thanks. >> >> diff --git a/mm/slub.c b/mm/slub.c >> index 2b1ce69..6adab87 100644 >> --- a/mm/slub.c >> +++ b/mm/slub.c >> @@ -2058,6 +2058,21 @@ static void put_cpu_partial(struct kmem_cache >> *s, struct page *page, int drain) >> >> } while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page) >> != oldpage); >> + >> + if (memcg_cache_dead(s)) { >> + bool done = false; >> + unsigned long flags; > > Suppose we are preempted here. In the meanwhile all objects are freed to > the cache, all frozen pages are unfrozen and also freed. The cache > destruction is then scheduled (patch 2 of this set). Then when this > thread continues execution it will operate on the cache that was > destroyed - use-after-free. > > I admit, this is very unlikely, but can we ignore this possibility? > Hello, >From your comment, now, I realize that your cache destruction solution has severe problem. With you solution, kmem_cache can be destroyed before last kfree() caller has returned. It means that we can't safely do anything related to the kmem_cache after losing control about that slab where we try to free object in free path. Consider __slab_free(). After put_cpu_partial() in __slab_free() is called, we attempt to update stat. There is possibility that this operation could be use-after-free with your solution. Until now, we have just stat operation, but it could be more. I don't like to impose this constraint to the slab free path. So IMHO, it is better that we should defer to destroy kmem_cache until last kfree() caller returns. Is it fair enough? :) Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/