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(). 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? -- Thanks, David / dhildenb
