From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx204.postini.com [74.125.245.204]) by kanga.kvack.org (Postfix) with SMTP id 911936B0039 for ; Wed, 7 Aug 2013 19:41:51 -0400 (EDT) Subject: Re: [PATCH] aoe: adjust ref of head for compound page tails MIME-Version: 1.0 (Apple Message framework v1085) Content-Type: text/plain; charset="us-ascii" From: Ed Cashin In-Reply-To: <20130807142755.5cd89e02e4286f7dca88b80d@linux-foundation.org> Date: Wed, 7 Aug 2013 19:41:48 -0400 Content-Transfer-Encoding: quoted-printable Message-ID: <3F0FBDD9-129C-45F4-A20C-3EB2E8EFC9C8@coraid.com> References: <0c8aff39249c1da6b9cc3356650149d065c3ebd2.1375320764.git.ecashin@coraid.com> <20130807135804.e62b75f6986e9568ab787562@linux-foundation.org> <8DFEA276-4EE1-44B4-9669-5634631D7BBC@coraid.com> <20130807141835.533816143f8b37175c50d58d@linux-foundation.org> <20130807142755.5cd89e02e4286f7dca88b80d@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Christoph Hellwig , linux-mm@kvack.org On Aug 7, 2013, at 5:27 PM, Andrew Morton wrote: > On Wed, 7 Aug 2013 14:18:35 -0700 Andrew Morton = wrote: >=20 >> On Wed, 7 Aug 2013 17:12:36 -0400 Ed Cashin = wrote: >>=20 >>>=20 >>> On Aug 7, 2013, at 4:58 PM, Andrew Morton wrote: >>>=20 >>>> On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin = wrote: >>>>=20 >>>>> As discussed previously, >>>>=20 >>>> I think I missed that. >>>>=20 >>>>> the fact that some users of the block >>>>> layer provide bios that point to pages with a zero _count means >>>>> that it is not OK for the network layer to do a put_page on the >>>>> skb frags during an skb_linearize, so the aoe driver gets a >>>>> reference to pages in bios and puts the reference before ending >>>>> the bio. And because it cannot use get_page on a page with a >>>>> zero _count, it manipulates the value directly. >>>>=20 >>>> Eh? What code is putting count=3D=3D0 pages into bios? That = sounds very >>>> weird and broken. >>>=20 >>> I thought so in 2007 but couldn't solicit a clear "this is wrong" = consensus from the discussion. >>>=20 >>> http://article.gmane.org/gmane.linux.kernel/499197 >>> https://lkml.org/lkml/2007/1/19/56 >>> https://lkml.org/lkml/2006/12/18/230 >>>=20 >>> We were seeing zero-count pages in bios from XFS, but Christoph = Hellwig pointed out that kmalloced pages can also come from ext3 when = it's doing log recovery, and they'll have zero page counts. >>=20 >> aiiee! >>=20 >> It is (I suppose) reasonable to put kmalloced memory into a BIO's = page >> array. And it is perfectly reasonable for a user of that bio to do a >> get_page/put_page against that page. It is utterly unreasonable for >> the damn page to get freed as a result! >>=20 >> I'd claim that slab is broken. The page is in use, so it should have = an >> elevated refcount, full stop. >>=20 >=20 > err, no. slab.c uses alloc_pages(), so the underlying page indeed has > a proper refcount. I'm still not understanding how this situation = comes > about. It sounds like it's wrong to give block pages with a zero count, so why = not just have aoe BUG_ON(compound_trans_head(bv->page->_count) =3D=3D 0) = until we're sure nobody does that anymore? If that idea makes sense to you, I will submit a new patch to follow the = one under discussion. --=20 Ed Cashin ecashin@coraid.com -- 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