On 29 Apr 2024, at 10:45, Zi Yan wrote: > On 29 Apr 2024, at 5:29, Ryan Roberts wrote: > >> On 27/04/2024 20:11, 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: >> >> Hi All, >> >> Thanks for all the feedback! I had thought that this patch would be entirely >> uncontraversial - obviously I was wrong :) >> >> I've read all the emails, and trying to summarize a way forward here... >> >>> >>> 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. >> >> I know its only semantics, but I don't view this as a new design constraint. I >> see it as an existing constraint that was previously being violated, and this >> patch aims to fix that. The generic version of pmdp_invalidate() unconditionally >> does a tlb invalidation on the address range covered by the pmd. That makes no >> sense unless the pmd was previously present. So my conclusion is that the >> function only expects to be called for present pmds. >> >> Additionally Documentation/mm/arch_pgtable_helpers.rst already says this: >> >> " >> | pmd_mkinvalid | Invalidates a mapped PMD [1] | >> " >> >> I read "mapped" to be a synonym for "present". So I think its already >> documented. Happy to explcitly change "mapped" to "present" though, if it helps? >> >> Finally, [1] which is linked from Documentation/mm/arch_pgtable_helpers.rst, >> also implies this constraint, although it doesn't explicitly say it. >> >> [1] https://lore.kernel.org/linux-mm/20181017020930.GN30832@xxxxxxxxxx/ >> >>> >>> 3.a) See the VM_WARN_ON() hunks below. >> >> It sounds like everybody would be happy if I sprinkle these into the arches that >> override pmdp_invalidate[_ad]()? There are 3 arches that have their own version >> of pmdp_invalidate(); powerpc, s390 and sparc. And 1 that has its own version of >> pmdp_invalidate_ad(); x86. I'll add them in all of those. >> >> I'll use VM_WARN_ON_ONCE() as suggested by John. >> >> I'd rather not put it directly into pmd_mkinvalid() since that would set a >> precedent for adding them absolutely everywhere. (e.g. pte_mkdirty(), ...). > > I understand your concern here. I assume you also understand the potential issue > with this, namely it does not prevent one from using pmd_mkinvalid() improperly > and causing a bug and the bug might only appear on arm64. > >> >>> >>> 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? >> >> If I change: >> >> " >> | pmd_mkinvalid | Invalidates a mapped PMD [1] | >> " >> >> To: >> >> " >> | pmd_mkinvalid | Invalidates a present PMD; do not call for | >> | non-present pmd [1] | >> " >> >> Is that sufficient? (I'll do the same for pud_mkinvalid() too. > > Sounds good to me. > > Also, if you move pmdp_invalidate(), please move the big comment with it to > avoid confusion. Thanks. And the Fixes tag does not need to go back that far, since this only affects arm64, which enables thp migration at commit 53fa117bb33c ("arm64/mm: Enable THP migration"). -- Best Regards, Yan, Zi
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