From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f45.google.com (mail-wg0-f45.google.com [74.125.82.45]) by kanga.kvack.org (Postfix) with ESMTP id 237BF6B0038 for ; Thu, 14 May 2015 04:01:48 -0400 (EDT) Received: by wgnd10 with SMTP id d10so64266346wgn.2 for ; Thu, 14 May 2015 01:01:47 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id dg7si2393746wib.78.2015.05.14.01.01.46 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 14 May 2015 01:01:46 -0700 (PDT) Date: Thu, 14 May 2015 10:01:45 +0200 From: Michal Hocko 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> Sender: owner-linux-mm@kvack.org List-ID: To: Eric B Munson Cc: Michael Kerrisk , Andrew Morton , Linus Torvalds , David Rientjes , LKML , Linux API , linux-mm@kvack.org 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, 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