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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Stat-Signature: wizj45p8cptz85z3d45zfw3wmu4m9jio X-Rspam-User: X-Rspamd-Queue-Id: 40D661C0002 X-HE-Tag: 1714371476-632330 X-HE-Meta: U2FsdGVkX1/sRoBP592ooAlH9idft0NIaIHde6UTPBemyVR+ufyb+6sH/vCC8zaPSPZ+gum5OAFO32Zd2hjl7K7Ttox18V0jwKxamk9vIlxTRk+oUouGgDXm4WyP8MGG8mg2xl+w2MHJ2l9qgDT85t1HSfOFKVrCXzFyiog/5TpF4zXXkCyq1mQizQpHJldKLYCov31fvSGrzOmi9H5I2BPVdi8p3cgW15TSodfw1byAB65XaMDDdkTR0yoUzo+VgGYpPqWbniId/TBQ0ben9OxIPi2pBaq3/+cgEyR930O+AAnZC85iWqunaPaRmBA7khvnzr038/XaHhq+rBNR/jCH67n3Oc17RMkaAbMfr0wjiignE3s4MCLCAN9vTWegwH2cN3yNxaGBLK4IfLBAMdC2vR3nXB6m+SvHSr5qty1NHCrDEs6T2ug463fAl1KlsoDVngjk73U2KL2vl1jhFNdS8wewfyggVS7G5WSZzsTjds1Xijl2HdRXVR2xPFRuXsQUD0DHmNPkZd3rRbNEs8jp/afvB2BgOh4lmpe5qMzcgjIIDGDTsxSAsm4mA/t8RhcFcc7jYi0DiLRaXkANbOvA3EZF78XaiFv9BaalSK1NRuNDJcRDWmCkGxNU30y7ScxI6NLUy3iCBJy2a0Jgg4Ps5/+cG5NN8n2RZ2onL8ZLoJdAqZrDHFBLE90VrhqJbSk/hHXpLBv0BusW16+HIyhI9yj6iolRXh11OImI/+CS3pBQpYkd8Z8ZzUt0R/oBuyOTSeZJoBjOc6Orakujp16DjsFKxZiNv8IeJmmOwT31OUg2iBl3KJU/xW1V0EUmuAIElGaoRjrsdTyebLxdSuDGdqqqs/S3WP/hktCLCAt0snrZmMPsTCy5W9A+YM6lE9aspPdPuj02r5ifJvsTUdVifDhA96wytjJPMy64dCR5++wFmdAcKH5+Ufr24BZ1UFz6KJ4P9RkOxlIM6j1 tZ/Qk3dd 8vFMm5nStpLqUDT5dvFuf8hz3JHnmWhX9+0SLKwQDyGb9FgCzcOxvc3K9gsgGDueGDeltK4qJ+QRvgh4IXjaCsIa0czcmD1WKpkJmZk/ucA1NPi0mrry7Ha5tgQZX5P7x3d+jchA+kLE1w754wQVahHIar6V8T9jsz7bq8P4bJ+XRjLRbIm47neMTsd66qlY6Yloj 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: List-Subscribe: List-Unsubscribe: On 4/28/24 02:18, Zi Yan wrote: > On 27 Apr 2024, at 16:45, Zi Yan wrote: > >> On 27 Apr 2024, at 15:11, John Hubbard wrote: >> >>> On 4/27/24 8:14 AM, Zi Yan wrote: >>>> On 27 Apr 2024, at 0:41, John Hubbard wrote: >>>>> On 4/25/24 10:07 AM, Ryan Roberts wrote: >>>>>> __split_huge_pmd_locked() can be called for a present THP, devmap or >>>>>> (non-present) migration entry. It calls pmdp_invalidate() >>>>>> unconditionally on the pmdp and only determines if it is present or not >>>>>> based on the returned old pmd. This is a problem for the migration entry >>>>>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be >>>>>> called for a present pmd. >>>>>> >>>>>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any >>>>>> future call to pmd_present() will return true. And therefore any >>>>>> lockless pgtable walker could see the migration entry pmd in this state >>>>>> and start interpretting the fields as if it were present, leading to >>>>>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker. >>>>>> I suspect the same is possible on other architectures. >>>>>> >>>>>> Fix this by only calling pmdp_invalidate() for a present pmd. And for >>>>> Yes, this seems like a good design decision (after reading through the >>>>> discussion that you all had in the other threads). >>>> This will only be good for arm64 and does not prevent other arch developers >>>> to write code breaking arm64, since only arm64's pmd_mkinvalid() can turn >>>> a swap entry to a pmd_present() entry. >>> Well, let's characterize it in a bit more detail, then: >>> >>> 1) This patch will make things better for arm64. That's important! >>> >>> 2) Equally important, this patch does not make anything worse for >>> other CPU arches. >>> >>> 3) This patch represents a new design constraint on the CPU arch >>> layer, and thus requires documentation and whatever enforcement >>> we can provide, in order to keep future code out of trouble. >>> >>> 3.a) See the VM_WARN_ON() hunks below. >>> >>> 3.b) I like the new design constraint, because it is reasonable and >>> clearly understandable: don't invalidate a non-present page >>> table entry. >>> >>> I do wonder if there is somewhere else that this should be documented? > In terms of documentation, at least in Documentation/mm/arch_pgtable_helpers.rst, > pmd_mkinvalid() entry needs to add "do not call on an invalid entry as > it breaks arm64" s/invalid/non-present ? ^^^^^^^^^^^^^ But validation via mm/debug_vm_pgtable.c would require a predictable return value from pmd_mkinvalid() e.g return old pmd when the entry is not present. ASSERT(pmd = pmd_mkinvalid(pmd)) - when pmd is not present Otherwise, wondering how the semantics could be validated in the test.