Re: [PATCH] mm: synchronize with PTE table reclaim for all folio sizes

[Baolin Wang]

On 11/5/26 6:06 PM, David Hildenbrand (Arm) wrote:
> On 11/5/26 10:00, Baolin Wang wrote:
> > CC Barry, who added the TTU_SYNC in shrink_folio_list().
> >
> > On 11/5/26 1:48 PM, Lance Yang wrote:
> > > On 2026/11/5 12:54, Lance Yang wrote:
> > > > Good catch!
> > > >
> > > >
> > > > Right. Whether the folio can be PMD-mapped doesn't tell us whether its
> > > > PTE table can be reclaimed, so those extra checks can skip synchronization
> > > > that the caller explicitly asked for ;)
> > >
> > > Ouch ... one thing I missed ... what about order-0 reclaim?
> > >
> > > shrink_folio_list() only sets TTU_SYNC for large folio, so can't we still
> > > hit the same race there?
> >
> > Thanks Lance. But this matches the purpose of the TTU_SYNC flag, which is used
> > to avoid racy checks with PVMW_SYNC. If no TTU_SYNC flag is set, it means the
> > caller can tolerate some racy checks.
> >
> > For this case, I think we should set TTU_SYNC unconditionally in
> > shrink_folio_list(). Barry, what do you think?
>
> That makes sense. I hope this doesn't throw a big wrench into our reclaim
> performance numbers.
>
> Can someone benchmark this?

Sure. I'll measure it if no one objects to this change.

> But maybe we should really follow-up with the folio-size based optimization in
> sync_with_folio_pmd_zap(), where we only need a memory barrier (and sufficient
> documentation why something that cannot be PMD-mapped can get away cheaper).

OK. I'll do this later.