Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965292AbbEMOsc (ORCPT ); Wed, 13 May 2015 10:48:32 -0400 Received: from prod-mail-xrelay02.akamai.com ([72.246.2.14]:50111 "EHLO prod-mail-xrelay02.akamai.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933344AbbEMOsb (ORCPT ); Wed, 13 May 2015 10:48:31 -0400 Date: Wed, 13 May 2015 10:48:30 -0400 From: Eric B Munson To: Michal Hocko Cc: Michael Kerrisk , Andrew Morton , Linus Torvalds , David Rientjes , LKML , Linux API , linux-mm@kvack.org, Michal Hocko Subject: Re: [PATCH 1/2] mmap.2: clarify MAP_LOCKED semantic Message-ID: <20150513144830.GF1227@akamai.com> 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: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="dWYAkE0V1FpFQHQ3" Content-Disposition: inline In-Reply-To: <20150513144506.GD1227@akamai.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3056 Lines: 78 --dWYAkE0V1FpFQHQ3 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, 13 May 2015, Eric B Munson wrote: > On Wed, 13 May 2015, Michal Hocko wrote: >=20 > > From: Michal Hocko > >=20 > > 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. > >=20 > > 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 somethi= ng > > else than the originally mapped area. > >=20 > > 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. > >=20 > > 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. > >=20 > > Signed-off-by: Michal Hocko >=20 > 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). >=20 > Either way this is a good catch. >=20 > Acked-by: Eric B Munson >=20 Sorry for the noise, this should have been a Reviewed-by: Eric B Munson --dWYAkE0V1FpFQHQ3 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBAgAGBQJVU2Q+AAoJELbVsDOpoOa9pw0QAJNQUeH65rfob8aNYHt8BEJX 5PJjFCHF0JkM8RR6CQUyFmZST5c/+G1oHY0DpoT5sWYFFoAFx0WziiIs1y3QC5tW gQoZXTY/jvlm9z8F6pcZKEnMgMTNtkvhjcP4/4uYXfB6dCcBbv3NmuWqpGjCal2W IiHZUwroFS14kIfcEr5WHfpuQAeSt4kEy2C8E/T92e3WZ9Gqks8x1T8OyVTQitxw 7N58hqndfG5tBTgHOIwsIfuxyvYvCnTg+gRWyoWsBaSbMOTDj/v9EWZCe7HDX78u eVJWNrpqx53TdNzx2stk4pwwGtkFOlv9Twq82ezgm/ByYKfBpUPMJk1XyxOsUkwn KOjmDMFIVaMCviCLBvqJL1B4gKK5zz10HkhndDj2LRr2LDQOcwhqNaBlvFOPnboZ lqou163GbbiJxC7hm1uySQJW+slYvy0d2g+rYoZ9WFKWfBIyrQKb3O/VR4hyojkB oqLcZpY9cFVP1nft7cjJEvKWYNIXrkrpOZ+VkkMcFSqeiGj0GQNCnCHIPQn+jwvP lZpcYtvfqGqkmMM2fXZFFFQ5ZSeYiGL2bWocbQXn3+wdq1YqYvbsw7VtHLtSO7XB c70vrUpnmfdvjS3mg5TsxapxyE07T0SQqMCjY07sfAodZbKy6IUQORza8ut+TV3S csaAt+4Olc28v/8N79tC =KLpL -----END PGP SIGNATURE----- --dWYAkE0V1FpFQHQ3-- -- 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/