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. -- Best Regards, Yan, Zi
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