From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx117.postini.com [74.125.245.117]) by kanga.kvack.org (Postfix) with SMTP id 675896B0032 for ; Wed, 7 Aug 2013 17:27:57 -0400 (EDT) Date: Wed, 7 Aug 2013 14:27:55 -0700 From: Andrew Morton Subject: Re: [PATCH] aoe: adjust ref of head for compound page tails Message-Id: <20130807142755.5cd89e02e4286f7dca88b80d@linux-foundation.org> In-Reply-To: <20130807141835.533816143f8b37175c50d58d@linux-foundation.org> References: <0c8aff39249c1da6b9cc3356650149d065c3ebd2.1375320764.git.ecashin@coraid.com> <20130807135804.e62b75f6986e9568ab787562@linux-foundation.org> <8DFEA276-4EE1-44B4-9669-5634631D7BBC@coraid.com> <20130807141835.533816143f8b37175c50d58d@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Ed Cashin , linux-kernel@vger.kernel.org, Christoph Hellwig , linux-mm@kvack.org On Wed, 7 Aug 2013 14:18:35 -0700 Andrew Morton wrote: > On Wed, 7 Aug 2013 17:12:36 -0400 Ed Cashin wrote: > > > > > On Aug 7, 2013, at 4:58 PM, Andrew Morton wrote: > > > > > On Thu, 1 Aug 2013 21:29:59 -0400 Ed Cashin wrote: > > > > > >> As discussed previously, > > > > > > I think I missed that. > > > > > >> 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. > > > > > > Eh? What code is putting count==0 pages into bios? That sounds very > > > weird and broken. > > > > I thought so in 2007 but couldn't solicit a clear "this is wrong" consensus from the discussion. > > > > http://article.gmane.org/gmane.linux.kernel/499197 > > https://lkml.org/lkml/2007/1/19/56 > > https://lkml.org/lkml/2006/12/18/230 > > > > 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. > > aiiee! > > 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! > > I'd claim that slab is broken. The page is in use, so it should have an > elevated refcount, full stop. > 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. -- 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