From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5A263CD4F42 for ; Fri, 22 Sep 2023 09:36:36 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4RsRwy6yv5z3dhy for ; Fri, 22 Sep 2023 19:36:34 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=ryan.roberts@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4RsRwP6cckz3cgv for ; Fri, 22 Sep 2023 19:36:02 +1000 (AEST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4668EDA7; Fri, 22 Sep 2023 02:36:08 -0700 (PDT) Received: from [10.57.65.11] (unknown [10.57.65.11]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A26293F5A1; Fri, 22 Sep 2023 02:35:26 -0700 (PDT) Message-ID: <9e8d66fb-1d8d-4ce0-86a7-4c8b04557cca@arm.com> Date: Fri, 22 Sep 2023 10:35:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 8/8] arm64: hugetlb: Fix set_huge_pte_at() to work with all swap entries Content-Language: en-GB To: Qi Zheng References: <20230921162007.1630149-1-ryan.roberts@arm.com> <20230921162007.1630149-9-ryan.roberts@arm.com> <217bb956-b9f6-1057-914b-436d4c775a8b@bytedance.com> <3358e732-8df9-4408-8249-384b102f5d75@arm.com> <6db7e7e0-4db6-f742-436b-1f4d8ae4e490@bytedance.com> From: Ryan Roberts In-Reply-To: <6db7e7e0-4db6-f742-436b-1f4d8ae4e490@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Catalin Marinas , Peter Xu , "James E.J. Bottomley" , linux-mm@kvack.org, sparclinux@vger.kernel.org, Alexander Gordeev , Will Deacon , linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, Vasily Gorbik , Helge Deller , Christoph Hellwig , Axel Rasmussen , Gerald Schaefer , Christian Borntraeger , Albert Ou , Arnd Bergmann , Anshuman Khandual , Heiko Carstens , Nicholas Piggin , Paul Walmsley , linux-arm-kernel@lists.infradead.org, SeongJae Park , Lorenzo Stoakes , linux-parisc@vger.kernel.org, Muchun Song , linux-kernel@vger.kernel .org, stable@vger.kernel.org, Uladzislau Rezki , Palmer Dabbelt , Sven Schnelle , Andrew Morton , linuxppc-dev@lists.ozlabs.org, "David S. Miller" , Mike Kravetz Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 22/09/2023 08:54, Qi Zheng wrote: > Hi Ryan, > > On 2023/9/22 15:40, Ryan Roberts wrote: >> On 22/09/2023 03:54, Qi Zheng wrote: >>> Hi Ryan, >>> >>> On 2023/9/22 00:20, Ryan Roberts wrote: >>>> When called with a swap entry that does not embed a PFN (e.g. >>>> PTE_MARKER_POISONED or PTE_MARKER_UFFD_WP), the previous implementation >>>> of set_huge_pte_at() would either cause a BUG() to fire (if >>>> CONFIG_DEBUG_VM is enabled) or cause a dereference of an invalid address >>>> and subsequent panic. >>>> >>>> arm64's huge pte implementation supports multiple huge page sizes, some >>>> of which are implemented in the page table with contiguous mappings. So >>>> set_huge_pte_at() needs to work out how big the logical pte is, so that >>>> it can also work out how many physical ptes (or pmds) need to be >>>> written. It does this by grabbing the folio out of the pte and querying >>>> its size. >>>> >>>> However, there are cases when the pte being set is actually a swap >>>> entry. But this also used to work fine, because for huge ptes, we only >>>> ever saw migration entries and hwpoison entries. And both of these types >>>> of swap entries have a PFN embedded, so the code would grab that and >>>> everything still worked out. >>>> >>>> But over time, more calls to set_huge_pte_at() have been added that set >>>> swap entry types that do not embed a PFN. And this causes the code to go >>>> bang. The triggering case is for the uffd poison test, commit >>>> 99aa77215ad0 ("selftests/mm: add uffd unit test for UFFDIO_POISON"), >>>> which sets a PTE_MARKER_POISONED swap entry. But review shows there are >>>> other places too (PTE_MARKER_UFFD_WP). >>>> >>>> So the root cause is due to commit 18f3962953e4 ("mm: hugetlb: kill >>>> set_huge_swap_pte_at()"), which aimed to simplify the interface to the >>>> core code by removing set_huge_swap_pte_at() (which took a page size >>>> parameter) and replacing it with calls to set_huge_swap_pte_at() where >>>> the size was inferred from the folio, as descibed above. While that >>>> commit didn't break anything at the time, >>> >>> If it didn't break anything at that time, then shouldn't the Fixes tag >>> be added to this commit? >>> >>>> it did break the interface >>>> because it couldn't handle swap entries without PFNs. And since then new >>>> callers have come along which rely on this working. >>> >>> So the Fixes tag should be added only to the commit that introduces the >>> first new callers? >> >> Well I guess it's a matter of point of view; My view is that 18f3962953e4 is the >> buggy change because it broke the interface to not be able to handle swap >> entries which do not contain PFNs. The fact that there were no callers that used >> the interface in this way at the time of the commit is irrelevant in my view. > > I understand your point of view. > > But IIUC, the Fixes tag is used to indicate the version that needs to > backport, but the version where the commit 18f3962953e4 is located > does not need to backport this bugfix patch. > >> But I already added 2 fixes tags; one for the buggy commit, and the other for >> the commit containing the new user of the interface. > > I think 2 fixes tags will cause inconvenience to the maintainers. > I did some Archaeology: $ git rev-list --no-walk=sorted --pretty=oneline \ 05e90bd05eea33fc77d6b11e121e2da01fee379f \ 60dfaad65aa97fb6755b9798a6b3c9e79bcd5930 \ 8a13897fb0daa8f56821f263f0c63661e1c6acae \ 18f3962953e40401b7ed98e8524167282c3e626e \ v6.5 v5.18 v5.17 v5.19 v6.5-rc6 v6.5-rc7 2dde18cd1d8fac735875f2e4987f11817cc0bc2c Linux 6.5 706a741595047797872e669b3101429ab8d378ef Linux 6.5-rc7 8a13897fb0daa8f56821f263f0c63661e1c6acae mm: userfaultfd: support UFFDIO_POISON for hugetlbfs 2ccdd1b13c591d306f0401d98dedc4bdcd02b421 Linux 6.5-rc6 3d7cb6b04c3f3115719235cc6866b10326de34cd Linux 5.19 18f3962953e40401b7ed98e8524167282c3e626e mm: hugetlb: kill set_huge_swap_pte_at() 4b0986a3613c92f4ec1bdc7f60ec66fea135991f Linux 5.18 05e90bd05eea33fc77d6b11e121e2da01fee379f mm/hugetlb: only drop uffd-wp special pte if required 60dfaad65aa97fb6755b9798a6b3c9e79bcd5930 mm/hugetlb: allow uffd wr-protect none ptes f443e374ae131c168a065ea1748feac6b2e76613 Linux 5.17 So it turns out that the PTE_MARKER_UFFD_WP markers were added first, using set_huge_pte_at(). At the time, this should have used set_huge_swap_pte_at(), so was arguably buggy for that reason. However, arm64 does not support UFFD_WP so none of the call sites that set the PTE_MARKER_UFFD_WP marker to the pte ever trigger on arm64. Then "mm: hugetlb: kill set_huge_swap_pte_at()" came along and "broke" the interface, but there were no callers relying on the behavoir that was broken. Then "mm: userfaultfd: support UFFDIO_POISON for hugetlbfs" came along in v6.5-rc7 and started relying on the broken behaviour of set_huge_pte_at(). So on that basis, I agree that the first commit where broken behaviour is observable is "mm: userfaultfd: support UFFDIO_POISON for hugetlbfs". So I will tag that one as "Fixes". (Although if set_huge_pte_at() was an exported symbol, then we would want to mark "mm: hugetlb: kill set_huge_swap_pte_at()"). Thanks, Ryan > Thanks, > Qi > >> >>> >>> Other than that, LGTM. >> >> Thanks! >> >>> >>> Thanks, >>> Qi >>>