On 31.08.22 21:34, Yang Shi wrote: > 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. ... or find another way to detect and handle this in GUP-fast? Not sure if, for handling PMDs, it could be sufficient to propagate the pmdp pointer + value and double check that the values didn't change. -- Thanks, David / dhildenb
