On 29/04/2024 16:29, Zi Yan wrote: > 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"). Yes, will do - good point. > > -- > Best Regards, > Yan, Zi
