On 31.08.22 21:08, Yang Shi wrote: > On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> On 31.08.22 19:55, Yang Shi wrote: >>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >>>> >>>> The comment is stale, because a TLB flush is no longer sufficient and >>>> required to synchronize against concurrent GUP-fast. This used to be true >>>> in the past, whereby a TLB flush would have implied an IPI on architectures >>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts >>>> from completing before completing the flush. >>> >>> Hmm... it seems there might be problem for THP collapse IIUC. THP >>> collapse clears and flushes pmd before doing anything on pte and >>> relies on interrupt disable of fast GUP to serialize against fast GUP. >>> But if TLB flush is no longer sufficient, then we may run into the >>> below race IIUC: >>> >>> CPU A CPU B >>> THP collapse fast GUP >>> >>> gup_pmd_range() <-- see valid pmd >>> >>> gup_pte_range() <-- work on pte >>> clear pmd and flush TLB >>> __collapse_huge_page_isolate() >>> isolate page <-- before GUP bump refcount >>> >>> pin the page >>> __collapse_huge_page_copy() >>> copy data to huge page >>> clear pte (don't flush TLB) >>> Install huge pmd for huge page >>> >>> return the obsolete page >> >> Hm, the is_refcount_suitable() check runs while the PTE hasn't been >> cleared yet. And we don't check if the PMD changed once we're in >> gup_pte_range(). > > Yes > >> >> The comment most certainly should be stale as well -- unless there is >> some kind of an implicit IPI broadcast being done. >> >> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an >> IPI broadcast on THP split (which is a rare event), allows one to >> protect a page table walker by merely disabling the interrupts during >> the walk." >> >> I'm not able to quickly locate that IPI broadcast -- maybe there is one >> being done here (in collapse) as well? > > The TLB flush may call IPI. I'm supposed it is arch dependent, right? > Some do use IPI, some may not. Right, and the whole idea of the RCU GUP-fast was to support architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do it -- but maybe it does so for PMDs? -- Thanks, David / dhildenb
