Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page()

[Peter Xu]

On Wed, Aug 31, 2022 at 10:55:43AM -0700, 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

Maybe the pmd level tlb flush is still needed, but on pte level it's
optional (where we can rely on fast-gup rechecking on the pte change)?

-- 
Peter Xu