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Wed, 20 Jun 2018 10:12:59 -0700 (PDT) From: Nadav Amit Message-Id: <3FA297CB-2475-498A-8372-3955FBB87AE1@gmail.com> Content-Type: multipart/signed; boundary="Apple-Mail=_D08CC230-0835-452A-83AB-719FA45AFAB1"; protocol="application/pgp-signature"; micalg=pgp-sha512 Mime-Version: 1.0 (Mac OS X Mail 11.4 \(3445.8.2\)) Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Date: Wed, 20 Jun 2018 10:12:56 -0700 In-Reply-To: <20180620071817.GJ13685@dhcp22.suse.cz> Cc: Yang Shi , Matthew Wilcox , ldufour@linux.vnet.ibm.com, Andrew Morton , Peter Zijlstra , Ingo Molnar , acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, "open list:MEMORY MANAGEMENT" , linux-kernel@vger.kernel.org To: Michal Hocko References: <1529364856-49589-1-git-send-email-yang.shi@linux.alibaba.com> <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com> <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com> <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com> <20180620071817.GJ13685@dhcp22.suse.cz> X-Mailer: Apple Mail (2.3445.8.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --Apple-Mail=_D08CC230-0835-452A-83AB-719FA45AFAB1 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 at 12:18 AM, Michal Hocko wrote: > On Tue 19-06-18 17:31:27, Nadav Amit wrote: >> at 4:08 PM, Yang Shi wrote: >>=20 >>> On 6/19/18 3:17 PM, Nadav Amit wrote: >>>> at 4:34 PM, Yang Shi >>>> wrote: >>>>=20 >>>>=20 >>>>> When running some mmap/munmap scalability tests with large memory = (i.e. >>>>>=20 >>>>>> 300GB), the below hung task issue may happen occasionally. >>>>> INFO: task ps:14018 blocked for more than 120 seconds. >>>>> Tainted: G E 4.9.79-009.ali3000.alios7.x86_64 #1 >>>>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this >>>>> message. >>>>> ps D 0 14018 1 0x00000004 >>>> (snip) >>>>=20 >>>>=20 >>>>> Zapping pages is the most time consuming part, according to the >>>>> suggestion from Michal Hock [1], zapping pages can be done with = holding >>>>> read mmap_sem, like what MADV_DONTNEED does. Then re-acquire write >>>>> mmap_sem to manipulate vmas. >>>> Does munmap() =3D=3D MADV_DONTNEED + munmap() ? >>>=20 >>> Not exactly the same. So, I basically copied the page zapping used = by munmap instead of calling MADV_DONTNEED. >>>=20 >>>> For example, what happens with userfaultfd in this case? Can you = get an >>>> extra #PF, which would be visible to userspace, before the munmap = is >>>> finished? >>>=20 >>> userfaultfd is handled by regular munmap path. So, no change to = userfaultfd part. >>=20 >> Right. I see it now. >>=20 >>>> In addition, would it be ok for the user to potentially get a = zeroed page in >>>> the time window after the MADV_DONTNEED finished removing a PTE and = before >>>> the munmap() is done? >>>=20 >>> This should be undefined behavior according to Michal. This has been = discussed in https://lwn.net/Articles/753269/. >>=20 >> Thanks for the reference. >>=20 >> Reading the man page I see: "All pages containing a part of the = indicated >> range are unmapped, and subsequent references to these pages will = generate >> SIGSEGV.=E2=80=9D >=20 > Yes, this is true but I guess what Yang Shi meant was that an = userspace > access racing with munmap is not well defined. You never know whether > you get your data, #PTF or SEGV because it depends on timing. The user > visible change might be that you lose content and get zero page = instead > if you hit the race window while we are unmapping which was not = possible > before. But whouldn't such an access pattern be buggy anyway? You need > some form of external synchronization AFAICS. It seems to follow the specifications, so it is not clearly buggy IMHO. = I don=E2=80=99t know of such a use-case, but if somebody does so - the = proposed change might even cause a security vulnerability. > But maybe some userspace depends on "getting right data or get SEGV" > semantic. If we have to preserve that then we can come up with a = VM_DEAD > flag set before we tear it down and force the SEGV on the #PF path. > Something similar we already do for MMF_UNSTABLE. That seems reasonable. Regards, Nadav --Apple-Mail=_D08CC230-0835-452A-83AB-719FA45AFAB1 Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEE0YCJM4pMIpzxUdmOK4dOkxJsY0AFAlsqixgACgkQK4dOkxJs Y0CLohAAzDqyXV2YQ9gTS3P5ug+gfWrJEIkshCcVsiybRqoVD0zqDRZHADnxpfa3 /yM5+HGNTizMA5+8/kYR0WF5iT0HudVxYwsCNWPFkroVKwFio5znLPQ9fl1ZHBah Gu+hFWP/TbkACKhn3TmnkvS8/7LkZlCvU+sBBGCo0++LH+rM6a+9E5T/+4h77f9v Ckjs9ncKlCvwI/zzTFa7aD7IDNUNSjzzC8asuytyGr6HrOjt8BOPSrkoknVa7d8P rw6nwi7v11Sni1ZkbhlomQus0EKCk+eJ6OxhMEqLHSQXVVqxZBNDVAR755iu0fwD aG5cseIaKSjycw23Go7iIYLS6ge2HRqEC5LX79wVfgLyjzKNQdEQu/kQpFIQgPYX kMNnrNmhHTEzmfYcR8vOgWYazInKJO5ECuCogg9SL/M1dHoSb7ocTdiBaBTS5M3i F/iKmBMzMFBf+r751wYDFkmFyvQVl6k/WXJvvvudB8U0D2hn6A4qBpt+tHj5LR4Q mnxlarsEfxnja/UEXdUktZsF8RrEVBLBC6lIvndX3qWP31coBcrk+dStN8FDklpI r/3pfIlK9ydfpVvYp6+wjuCBIMRqkVPQHHe8Xfv5GdlyOXQwWv8UnkMX95A8vUb5 TojMmwoL/FHqSUKW+kh17NQ2jG+WGEVUwIBFVPmqH9jP0KXAWYU= =hifR -----END PGP SIGNATURE----- --Apple-Mail=_D08CC230-0835-452A-83AB-719FA45AFAB1--