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Fri, 22 Sep 2023 02:58:25 -0700 (PDT) Received: from [10.84.155.178] ([203.208.167.146]) by smtp.gmail.com with ESMTPSA id lt22-20020a17090b355600b00276d039aecasm2933526pjb.13.2023.09.22.02.58.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 22 Sep 2023 02:58:24 -0700 (PDT) Message-ID: <41b6524a-effe-c600-962c-2b6e32526dc8@bytedance.com> Date: Fri, 22 Sep 2023 17:58:11 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v1 8/8] arm64: hugetlb: Fix set_huge_pte_at() to work with all swap entries Content-Language: en-US To: Ryan Roberts Cc: Catalin Marinas , Will Deacon , "James E.J. Bottomley" , Helge Deller , Nicholas Piggin , Christophe Leroy , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Gerald Schaefer , "David S. Miller" , Arnd Bergmann , Mike Kravetz , Muchun Song , SeongJae Park , Andrew Morton , Uladzislau Rezki , Christoph Hellwig , Lorenzo Stoakes , Anshuman Khandual , Peter Xu , Axel Rasmussen , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org 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> <9e8d66fb-1d8d-4ce0-86a7-4c8b04557cca@arm.com> From: Qi Zheng In-Reply-To: <9e8d66fb-1d8d-4ce0-86a7-4c8b04557cca@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: pheu9syd45tb6kpd3gmq65eqp4476z61 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: AA37F40018 X-HE-Tag: 1695376706-689937 X-HE-Meta: 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 OYplgbZC NLGRUen+ClpBrbXKsR/DJok2v2mYx6ldGEYfyzhTn6/lX3fm/JPOiFOnSaO53J3uCA3dtrkxltY0UuCJxmwo1yaslgmJLQKdHSuqPTptR/xgiTr7GsB/HruQ96pYSF/OJdk8FEiNo5T4/icUQs3qRlKZLhK5/EqlUVcCuQjFIg+pxfvuhvAl0swDzhYih7yFUI6hpC0g1vo8Dpv+Mdju9D5p04Bn7r1ABFe0DOTwjcMrMYAKUhnbC4joq2IGaDFfGRQLV6v5ZoSgtDYAO/9I8Mq7ULyQgqAZecilwmWt5Tew1b2DYw/qZ0U7SpdSLtTa6PEVsUkNnLuTZbmJ7sqNbwNEuekJZbDmPIawcNmXnHz9c2LEetlBhZJnplvHrki/3I7+JaBzffffzBEtVym3MEOeGRonNYzl7neeB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi Ryan, On 2023/9/22 17:35, Ryan Roberts wrote: > 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: Nice! Thanks for doing this. > > $ 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(). Got it. > > 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()"). Agree. I just checked the time point when 18f3962953e4 was added, neither set_huge_pte_at() nor set_huge_swap_pte_at() are exported symbols. Thanks, Qi > > Thanks, > Ryan > > > > >> Thanks, >> Qi >> >>> >>>> >>>> Other than that, LGTM. >>> >>> Thanks! >>> >>>> >>>> Thanks, >>>> Qi >>>>