On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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? It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC. So maybe we should implement pmdp_collapse_flush() for those arches to issue IPI. > > -- > Thanks, > > David / dhildenb >
