Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932320AbbENIBv (ORCPT ); Thu, 14 May 2015 04:01:51 -0400 Received: from cantor2.suse.de ([195.135.220.15]:52832 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751325AbbENIBr (ORCPT ); Thu, 14 May 2015 04:01:47 -0400 Date: Thu, 14 May 2015 10:01:45 +0200 From: Michal Hocko To: Eric B Munson Cc: Michael Kerrisk , Andrew Morton , Linus Torvalds , David Rientjes , LKML , Linux API , linux-mm@kvack.org Subject: Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic Message-ID: <20150514080145.GB6433@dhcp22.suse.cz> References: <1431527892-2996-1-git-send-email-miso@dhcp22.suse.cz> <1431527892-2996-2-git-send-email-miso@dhcp22.suse.cz> <20150513144506.GD1227@akamai.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150513144506.GD1227@akamai.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2124 Lines: 52 On Wed 13-05-15 10:45:06, Eric B Munson wrote: > On Wed, 13 May 2015, Michal Hocko wrote: > > > From: Michal Hocko > > > > MAP_LOCKED had a subtly different semantic from mmap(2)+mlock(2) since > > it has been introduced. > > mlock(2) fails if the memory range cannot get populated to guarantee > > that no future major faults will happen on the range. mmap(MAP_LOCKED) on > > the other hand silently succeeds even if the range was populated only > > partially. > > > > Fixing this subtle difference in the kernel is rather awkward because > > the memory population happens after mm locks have been dropped and so > > the cleanup before returning failure (munlock) could operate on something > > else than the originally mapped area. > > > > E.g. speculative userspace page fault handler catching SEGV and doing > > mmap(fault_addr, MAP_FIXED|MAP_LOCKED) might discard portion of a racing > > mmap and lead to lost data. Although it is not clear whether such a > > usage would be valid, mmap page doesn't explicitly describe requirements > > for threaded applications so we cannot exclude this possibility. > > > > This patch makes the semantic of MAP_LOCKED explicit and suggest using > > mmap + mlock as the only way to guarantee no later major page faults. > > > > Signed-off-by: Michal Hocko > > Does the problem still happend when MAP_POPULATE | MAP_LOCKED is used > (AFAICT MAP_POPULATE will cause the mmap to fail if all the pages cannot > be made present). No, there is no difference because MAP_POPULATE is implicit when MAP_LOCKED is used and as pointed in the cover, we cannot fail after the vma is created and locks dropped. The second patch tries to clarify that MAP_POPULATE is just a best effort. > Either way this is a good catch. > > Acked-by: Eric B Munson Thanks! -- Michal Hocko SUSE Labs -- 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/