Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754737AbbHXPJR (ORCPT ); Mon, 24 Aug 2015 11:09:17 -0400 Received: from a23-79-238-179.deploy.static.akamaitechnologies.com ([23.79.238.179]:54471 "EHLO prod-mail-xrelay05.akamai.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753772AbbHXPJP (ORCPT ); Mon, 24 Aug 2015 11:09:15 -0400 Date: Mon, 24 Aug 2015 11:09:12 -0400 From: Eric B Munson To: Vlastimil Babka Cc: Konstantin Khlebnikov , Michal Hocko , Andrew Morton , Jonathan Corbet , "Kirill A. Shutemov" , Linux Kernel Mailing List , dri-devel , "linux-mm@kvack.org" , Linux API Subject: Re: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT Message-ID: <20150824150912.GA17005@akamai.com> References: <20150812115909.GA5182@dhcp22.suse.cz> <20150819213345.GB4536@akamai.com> <20150820075611.GD4780@dhcp22.suse.cz> <20150820170309.GA11557@akamai.com> <20150821072552.GF23723@dhcp22.suse.cz> <20150821183132.GA12835@akamai.com> <55DB1C77.8070705@suse.cz> <55DB29EB.1000308@suse.cz> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="liOOAslEiF7prFVr" Content-Disposition: inline In-Reply-To: <55DB29EB.1000308@suse.cz> 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: 4545 Lines: 119 --liOOAslEiF7prFVr Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, 24 Aug 2015, Vlastimil Babka wrote: > On 08/24/2015 03:50 PM, Konstantin Khlebnikov wrote: > >On Mon, Aug 24, 2015 at 4:30 PM, Vlastimil Babka wrote: > >>On 08/24/2015 12:17 PM, Konstantin Khlebnikov wrote: > >>>> > >>>> > >>>>I am in the middle of implementing lock on fault this way, but I cann= ot > >>>>see how we will hanlde mremap of a lock on fault region. Say we have > >>>>the following: > >>>> > >>>> addr =3D mmap(len, MAP_ANONYMOUS, ...); > >>>> mlock(addr, len, MLOCK_ONFAULT); > >>>> ... > >>>> mremap(addr, len, 2 * len, ...) > >>>> > >>>>There is no way for mremap to know that the area being remapped was l= ock > >>>>on fault so it will be locked and prefaulted by remap. How can we av= oid > >>>>this without tracking per vma if it was locked with lock or lock on > >>>>fault? > >>> > >>> > >>>remap can count filled ptes and prefault only completely populated are= as. > >> > >> > >>Does (and should) mremap really prefault non-present pages? Shouldn't it > >>just prepare the page tables and that's it? > > > >As I see mremap prefaults pages when it extends mlocked area. > > > >Also quote from manpage > >: If the memory segment specified by old_address and old_size is locked > >: (using mlock(2) or similar), then this lock is maintained when the seg= ment is > >: resized and/or relocated. As a consequence, the amount of memory loc= ked > >: by the process may change. >=20 > Oh, right... Well that looks like a convincing argument for having a > sticky VM_LOCKONFAULT after all. Having mremap guess by scanning > existing pte's would slow it down, and be unreliable (was the area > completely populated because MLOCK_ONFAULT was not used or because > the process aulted it already? Was it not populated because > MLOCK_ONFAULT was used, or because mmap(MAP_LOCKED) failed to > populate it all?). Given this, I am going to stop working in v8 and leave the vma flag in place. >=20 > The only sane alternative is to populate always for mremap() of > VM_LOCKED areas, and document this loss of MLOCK_ONFAULT information > as a limitation of mlock2(MLOCK_ONFAULT). Which might or might not > be enough for Eric's usecase, but it's somewhat ugly. >=20 I don't think that this is the right solution, I would be really surprised as a user if an area I locked with MLOCK_ONFAULT was then fully locked and prepopulated after mremap(). > >> > >>>There might be a problem after failed populate: remap will handle them > >>>as lock on fault. In this case we can fill ptes with swap-like non-pre= sent > >>>entries to remember that fact and count them as should-be-locked pages. > >> > >> > >>I don't think we should strive to have mremap try to fix the inherent > >>unreliability of mmap (MAP_POPULATE)? > > > >I don't think so. MAP_POPULATE works only when mmap happens. > >Flag MREMAP_POPULATE might be a good idea. Just for symmetry. >=20 > Maybe, but please do it as a separate series. >=20 > -- > 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 --liOOAslEiF7prFVr Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBAgAGBQJV2zOYAAoJELbVsDOpoOa914QP/1WLYpDWRu7QcEFQsZ9/EphN U65FGIqpVmkV7VQrHdpYjotjrr4PpNP+Z8u1pv/QDarupqrbINuFaBF3ip9IN1bB kgDpUyFSK04nSRDMVF5jww7ONXjhr7d5NgT8wn+HXW5l34FZkFlHCDEldA/WI6yz IjPFlIfQRBxUgmiDcOFBNWLiacDLpJICQjshwdMTmZl7RedJM184lXS8mIL2CINs 4og3PWz6xDSAoJ3PfaOcVtfKPYAdQDct8/18fr5o2/7bVa+DXr4xQ/vJUsmqXpXc ZEz1ZDCAbiq4qS+ybPdWonhnl4nNxeid1HZGrxNEDA6kHlmZ8Vw5uh7vh8HtlEWJ RxMTGONvrVC8s2ijDOzqqfAD7+8zupBouomgYIoKLSyPsXMOvfqFJqO4JJoj+pmP i8Gz6OedSsXI28VoctkTyfVkCB4f1eYRARgXbxMhpC+eCL35trfIqA/a6kfxKu7c JrhJqND3cM2UdTmT6G/KVwnTzpoTxV7lfmgnM0/K61CIJ0/s2xGClwxiLu6lLdM/ bwxQuh6yVXCVccqN7eDBcrkRuabzcIUfbHu6YL1roahLb+gLKYyRgYrKdBmi6u2v PoEwOMvUqgiiZFfp3qBgRolssOgdW6G8SRWk2z9Rky9c2nMsCe01RP34YZUrlvWc GM8koujIdjNTzrk505ic =iivI -----END PGP SIGNATURE----- --liOOAslEiF7prFVr-- -- 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/