From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f169.google.com (mail-ie0-f169.google.com [209.85.223.169]) by kanga.kvack.org (Postfix) with ESMTP id C0EF16B0032 for ; Wed, 29 Apr 2015 20:28:57 -0400 (EDT) Received: by iedfl3 with SMTP id fl3so62614748ied.1 for ; Wed, 29 Apr 2015 17:28:57 -0700 (PDT) Received: from mail-ig0-x231.google.com (mail-ig0-x231.google.com. [2607:f8b0:4001:c05::231]) by mx.google.com with ESMTPS id j5si833333igh.50.2015.04.29.17.28.56 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 29 Apr 2015 17:28:57 -0700 (PDT) Received: by igblo3 with SMTP id lo3so63550492igb.0 for ; Wed, 29 Apr 2015 17:28:56 -0700 (PDT) Date: Wed, 29 Apr 2015 17:28:54 -0700 (PDT) From: David Rientjes Subject: Re: [RFC PATCH] mmap.2: clarify MAP_LOCKED semantic (was: Re: Should mmap MAP_LOCKED fail if mm_poppulate fails?) In-Reply-To: <20150429113818.GC16097@dhcp22.suse.cz> Message-ID: References: <20150114095019.GC4706@dhcp22.suse.cz> <1430223111-14817-1-git-send-email-mhocko@suse.cz> <20150428164302.GI2659@dhcp22.suse.cz> <20150428183535.GB30918@dhcp22.suse.cz> <20150429113818.GC16097@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Linus Torvalds , Michael Kerrisk , linux-mm , Cyril Hrubis , Andrew Morton , Hugh Dickins , Michel Lespinasse , Rik van Riel , LKML , Linux API On Wed, 29 Apr 2015, Michal Hocko wrote: > 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 > --- > 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 The wording of this begs the question on the behavior of MAP_LOCKED | MAP_POPULATE since this same man page specifies that accesses to memory mapped with MAP_POPULATE will not block on page faults later. I think Documentation/vm/unevictable-lru.txt would benefit from an update under the mmap(MAP_LOCKED) section where all this can be laid out and perhaps reference it from the man page? -- 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