Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936601AbcJRPcl (ORCPT ); Tue, 18 Oct 2016 11:32:41 -0400 Received: from mail-wm0-f68.google.com ([74.125.82.68]:32895 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936195AbcJRPa4 (ORCPT ); Tue, 18 Oct 2016 11:30:56 -0400 Date: Tue, 18 Oct 2016 17:30:50 +0200 From: Michal Hocko To: Lorenzo Stoakes Cc: linux-mm@kvack.org, Linus Torvalds , Jan Kara , Hugh Dickins , Dave Hansen , Rik van Riel , Mel Gorman , Andrew Morton , adi-buildroot-devel@lists.sourceforge.net, ceph-devel@vger.kernel.org, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, kvm@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-cris-kernel@axis.com, linux-fbdev@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linux-mips@linux-mips.org, linux-rdma@vger.kernel.org, linux-s390@vger.kernel.org, linux-samsung-soc@vger.kernel.org, linux-scsi@vger.kernel.org, linux-security-module@vger.kernel.org, linux-sh@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 00/10] mm: adjust get_user_pages* functions to explicitly pass FOLL_* flags Message-ID: <20161018153050.GC13117@dhcp22.suse.cz> References: <20161013002020.3062-1-lstoakes@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161013002020.3062-1-lstoakes@gmail.com> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2638 Lines: 51 On Thu 13-10-16 01:20:10, Lorenzo Stoakes wrote: > This patch series adjusts functions in the get_user_pages* family such that > desired FOLL_* flags are passed as an argument rather than implied by flags. > > The purpose of this change is to make the use of FOLL_FORCE explicit so it is > easier to grep for and clearer to callers that this flag is being used. The use > of FOLL_FORCE is an issue as it overrides missing VM_READ/VM_WRITE flags for the > VMA whose pages we are reading from/writing to, which can result in surprising > behaviour. > > The patch series came out of the discussion around commit 38e0885, which > addressed a BUG_ON() being triggered when a page was faulted in with PROT_NONE > set but having been overridden by FOLL_FORCE. do_numa_page() was run on the > assumption the page _must_ be one marked for NUMA node migration as an actual > PROT_NONE page would have been dealt with prior to this code path, however > FOLL_FORCE introduced a situation where this assumption did not hold. > > See https://marc.info/?l=linux-mm&m=147585445805166 for the patch proposal. I like this cleanup. Tracking FOLL_FORCE users was always a nightmare and the flag behavior is really subtle so we should better be explicit about it. I haven't gone through each patch separately but rather applied the whole series and checked the resulting diff. This all seems OK to me and feel free to add Acked-by: Michal Hocko I am wondering whether we can go further. E.g. it is not really clear to me whether we need an explicit FOLL_REMOTE when we can in fact check mm != current->mm and imply that. Maybe there are some contexts which wouldn't work, I haven't checked. Then I am also wondering about FOLL_TOUCH behavior. __get_user_pages_unlocked has only few callers which used to be get_user_pages_unlocked before 1e9877902dc7e ("mm/gup: Introduce get_user_pages_remote()"). To me a dropped FOLL_TOUCH seems unintentional. Now that get_user_pages_unlocked has gup_flags argument I guess we might want to get rid of the __g-u-p-u version altogether, no? __get_user_pages is quite low level and imho shouldn't be exported. It's only user - kvm - should rather pull those two functions to gup instead and export them. There is nothing really KVM specific in them. I also cannot say I would be entirely thrilled about get_user_pages_locked, we only have one user which can simply do lock g-u-p unlock AFAICS. I guess there is more work in that area and I do not want to impose all that work on you, but I couldn't resist once I saw you playing in that area ;) Definitely a good start! -- Michal Hocko SUSE Labs