On 9 May 2024, at 4:21, Lance Yang wrote: > Hey Zi and Jason, > > Thanks a lot for reaching out! > > On Thu, May 9, 2024 at 12:35 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: >> >> On Wed, May 08, 2024 at 12:22:08PM -0400, Zi Yan wrote: >>> On 8 May 2024, at 11:52, Jason Gunthorpe wrote: >>> >>>> On Wed, May 08, 2024 at 10:56:34AM -0400, Zi Yan wrote: >>>> >>>>> Lance is improving try_to_unmap_one() to support unmapping PMD THP as a whole, >>>>> so he moves split_huge_pmd_address() inside while (page_vma_mapped_walk(&pvmw)) >>>>> and after mmu_notifier_invalidate_range_start() as split_huge_pmd_locked() >>>>> and does not include the mmu notifier ops inside split_huge_pmd_address(). > > IMO, It might be reasonable to exclude the mmu notifier ops in > split_huge_pmd_locked(). IIUC, before acquiring the PTL, callers need to tear > down the secondary mappings via mmu_notifier_invalidate_range_start() with > the range aligned to HPAGE_PMD_SIZE. > >>>>> I wonder if that could cause issues, since the mmu_notifier_invalidate_range_start() >>>>> before the while loop only has range of the original address and >>>>> split huge pmd can affect the entire PMD address range and these two ranges >>>>> might not be the same. > > As Baolin mentioned [1] before: > "For a PMD mapped THP, I think the address is already THP size alignment > returned from vma_address(&folio->page, vma)." > > Given this, perhaps we don't need to re-align the input address after > starting the pagewalk? IMO, if any corner cases arise, we should catch them > by using VM_WARN_ON_ONCE() in split_huge_pmd_locked(). > > Zi, what do you think? Yes, I agree. Thanks for sorting this out. > > [1] https://lore.kernel.org/linux-mm/cc9fd23f-7d87-48a7-a737-acbea8e95fb7@xxxxxxxxxxxxxxxxx/ > >>>> >>>> That does not sound entirely good.. >>>> >>>> I suppose it depends on what split does, if the MM page table has the >>>> same translation before and after split then perhaps no invalidation >>>> is even necessary. >>> >>> Before split, it is a PMD mapping to a PMD THP (order-9). After split, >>> they are 512 PTEs mapping to the same THP. Unless the secondary TLB >>> does not support PMD mapping and use 512 PTEs instead, it seems to >>> be an issue from my understanding. >> >> I may not recall fully, but I don't think any secondaries are >> so sensitive to the PMD/PTE distinction.. At least the ones using >> hmm_range_fault() are not. >> >> When the PTE eventually comes up for invalidation then the secondary >> should wipe out any granual they may have captured. >> >> Though, perhaps KVM should be checked carefully. >> >>> In terms of two mmu_notifier ranges, first is in the split_huge_pmd_address()[1] >>> and second is in try_to_unmap_one()[2]. When try_to_unmap_one() is unmapping >>> a subpage in the middle of a PMD THP, the former notifies about the PMD range >>> change due to one PMD split into 512 PTEs and the latter only needs to notify >>> about the invalidation of the unmapped PTE. I do not think the latter can >>> replace the former, although a potential optimization can be that the latter >>> can be removed as it is included in the range of the former. >> >> I think we probably don't need both, either size might be fine, but >> the larger size is definately fine.. >> >>> Regarding Lance's current code change, is it OK to change mmu_notifier range >>> after mmu_notifier_invalidate_range_start()? >> >> No, it cannot be changed during a start/stop transaction. > > I understood and will keep that in mind - thanks! > > Thanks again for clarifying! > Lance > >> >> Jason >> >> -- Best Regards, Yan, Zi
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