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(). > >> 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. > > > > 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. Jason
