On Wed, Aug 31, 2022 at 12:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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. Should work too, right before pinning the page. pmdp_collapse_flush() is actually just called by khugepaged, so arch specific implementation should not be a problem and we avoid making gup fast more complicated. > > -- > Thanks, > > David / dhildenb >
