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. > > -- > Thanks, > > David / dhildenb >
