From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id A5AEE6B0253 for ; Fri, 15 Dec 2017 04:36:21 -0500 (EST) Received: by mail-wr0-f199.google.com with SMTP id g12so776076wra.2 for ; Fri, 15 Dec 2017 01:36:21 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id v8si4983077wrv.242.2017.12.15.01.36.20 for (version=TLS1 cipher=AES128-SHA bits=128/128); Fri, 15 Dec 2017 01:36:20 -0800 (PST) Date: Fri, 15 Dec 2017 10:36:18 +0100 From: Michal Hocko Subject: Re: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages Message-ID: <20171215093618.GV16951@dhcp22.suse.cz> References: <1511780964-64864-1-git-send-email-chenjiankang1@huawei.com> <20171127113341.ldx32qvexqe2224d@dhcp22.suse.cz> <20171129160446.jluzpv3n6mjc3fwv@dhcp22.suse.cz> <20171129134159.c9100ea6dacad870d69929b7@linux-foundation.org> <20171130065335.zno7peunnl2zpozq@dhcp22.suse.cz> <20171130131706.0550cd28ce47aaa976f7db2a@linux-foundation.org> <20171201072414.3kc3pbvdbqbxhnfx@dhcp22.suse.cz> <20171201111845.iyoua7hhjodpuvoy@dhcp22.suse.cz> <20171214140608.GQ16951@dhcp22.suse.cz> <20171214123309.bdee142c82809f4c4ff3ce5b@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171214123309.bdee142c82809f4c4ff3ce5b@linux-foundation.org> Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: JianKang Chen , mgorman@techsingularity.net, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, xieyisheng1@huawei.com, guohanjun@huawei.com, wangkefeng.wang@huawei.com On Thu 14-12-17 12:33:09, Andrew Morton wrote: > On Thu, 14 Dec 2017 15:06:08 +0100 Michal Hocko wrote: > > > On Fri 01-12-17 12:18:45, Michal Hocko wrote: > > > On Fri 01-12-17 08:24:14, Michal Hocko wrote: > > > > On Thu 30-11-17 13:17:06, Andrew Morton wrote: > > > > > On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko wrote: > > > > > > > > > > > > mm... So we have a caller which hopes to be getting highmem pages but > > > > > > > isn't. Caller then proceeds to pointlessly kmap the page and wonders > > > > > > > why it isn't getting as much memory as it would like on 32-bit systems, > > > > > > > etc. > > > > > > > > > > > > How he can kmap the page when he gets a _virtual_ address? > > > > > > > > > > doh. > > > > > > > > > > > > I do think we should help ferret out such bogosity. A WARN_ON_ONCE > > > > > > > would suffice. > > > > > > > > > > > > This function has always been about lowmem pages. I seriously doubt we > > > > > > have anybody confused and asking for a highmem page in the kernel. I > > > > > > haven't checked that but it would already blow up as VM_BUG_ON tends to > > > > > > be enabled on many setups. > > > > > > > > > > OK. But silently accepting __GFP_HIGHMEM is a bit weird - callers > > > > > shouldn't be doing that in the first place. > > > > > > > > Yes, they shouldn't be. > > > > > > > > > I wonder what happens if we just remove the WARN_ON and pass any > > > > > __GFP_HIGHMEM straight through. The caller gets a weird address from > > > > > page_to_virt(highmem page) and usually goes splat? Good enough > > > > > treatment for something which never happens anyway? > > > > > > > > page_address will return NULL so they will blow up and leak the freshly > > > > allocated memory. > > > > > > let me be more specific. They will blow up and leak if the returned > > > address is not checked. If it is then we just leak. None of that sounds > > > good to me. > > > > So do we care and I will resend the patch in that case or I just drop > > this from my patch queue? > > Well.. I still think that silently accepting bad input would be bad > practice. If we can just delete the assertion and have such a caller > reliably blow up later on then that's good enough. The point is that if the caller checks for the failed allocation then the result is a memory leak. -- Michal Hocko SUSE Labs -- 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