From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f41.google.com (mail-wg0-f41.google.com [74.125.82.41]) by kanga.kvack.org (Postfix) with ESMTP id 9B3236B0038 for ; Thu, 14 May 2015 09:36:16 -0400 (EDT) Received: by wgnd10 with SMTP id d10so73371816wgn.2 for ; Thu, 14 May 2015 06:36:16 -0700 (PDT) Received: from mail-wg0-x22b.google.com (mail-wg0-x22b.google.com. [2a00:1450:400c:c00::22b]) by mx.google.com with ESMTPS id d8si38757761wjx.11.2015.05.14.06.36.14 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 14 May 2015 06:36:15 -0700 (PDT) Received: by wgbhc8 with SMTP id hc8so42194972wgb.3 for ; Thu, 14 May 2015 06:36:14 -0700 (PDT) Message-ID: <5554A4C6.9030306@gmail.com> Date: Thu, 14 May 2015 15:36:06 +0200 From: "Michael Kerrisk (man-pages)" MIME-Version: 1.0 Subject: Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic References: <1431527892-2996-1-git-send-email-miso@dhcp22.suse.cz> <1431527892-2996-2-git-send-email-miso@dhcp22.suse.cz> In-Reply-To: <1431527892-2996-2-git-send-email-miso@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: mtk.manpages@gmail.com, Andrew Morton , Linus Torvalds , David Rientjes , LKML , Linux API , linux-mm@kvack.org, Michal Hocko , Eric B Munson On 05/13/2015 04:38 PM, 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. Thanks, Michal. Applied, with Reviewed-by: from Eric added. Cheers, Michael > Signed-off-by: Michal Hocko > --- > man2/mmap.2 | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/man2/mmap.2 b/man2/mmap.2 > index 54d68cf87e9e..1486be2e96b3 100644 > --- a/man2/mmap.2 > +++ b/man2/mmap.2 > @@ -235,8 +235,19 @@ See the Linux kernel source file > for further information. > .TP > .BR MAP_LOCKED " (since Linux 2.5.37)" > -Lock the pages of the mapped region into memory in the manner of > +Mark the mmaped region to be locked in the same way as > .BR mlock (2). > +This implementation will try to populate (prefault) the whole range but > +the mmap call doesn't fail with > +.B ENOMEM > +if this fails. Therefore major faults might happen later on. So the semantic > +is not as strong as > +.BR mlock (2). > +.BR mmap (2) > ++ > +.BR mlock (2) > +should be used when major faults are not acceptable after the initialization > +of the mapping. > This flag is ignored in older kernels. > .\" If set, the mapped pages will not be swapped out. > .TP > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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