Re: [RFC PATCH] mm: only set fault addrsss' access bit in do_anonymous_page

[Kiryl Shutsemau <kirill@shutemov.name>]

On Wed, Feb 11, 2026 at 09:00:45AM +0800, Wenchao Hao wrote:
> On Tue, Feb 10, 2026 at 7:56 PM Kiryl Shutsemau <kirill@shutemov.name> wrote:
> >
> > On Tue, Feb 10, 2026 at 12:34:56PM +0800, Wenchao Hao wrote:
> > > When do_anonymous_page() creates mappings for huge pages, it currently sets
> > > the access bit for all mapped PTEs (Page Table Entries) by default.
> > >
> > > This causes an issue where the Referenced field in /proc/pid/smaps cannot
> > > distinguish whether a page was actually accessed.
> > >
> > > So here introduces a new interface, set_anon_ptes(), which only sets the
> > > access bit for the PTE corresponding to the faulting address. This allows
> > > accurate tracking of page access status in /proc/pid/smaps before memory
> > > reclaim scan the folios.
> > >
> > > During memory reclaim: folio_referenced() checks and clears the access bits
> > > of PTEs, rmap verifies all PTEs under a folio. If any PTE mapped subpage of
> > > folio has access bit set, the folio is retained during reclaim. So only
> > > set the access bit for the faulting PTE in do_anonymous_page() is safe, as
> > > it does not interfere with reclaim decisions.
> >
> > We had similar discussion about faultaround and briefly made it produce
> > old ptes, but it caused performance regression as old ptes require
> > additional pagewalk to set accessed bit on touch. It got reverted,
> > but arch can opt-in for setting up old ptes for non-fault address.
> >
> > See commits:
> >
> > 5c0a85fad949 ("mm: make faultaround produce old ptes")
> > 315d09bf30c2 ("Revert "mm: make faultaround produce old ptes"")
> > 46bdb4277f98 ("mm: Allow architectures to request 'old' entries when prefaulting")
> >
> It does look similar—our modifications both revolve around whether pre-mapped
> PTEs should be marked as "new."
> 
> Was there any analysis into why your changes led to performance regressions?

As I mentioned, my theory was that it is due to an additional pagewalks
CPU has to do to flip access bit when it touches the memory, but I
didn't profile it to confirm.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov