Re: [RFC PATCH v3 1/8] mm: multi-gen LRU: ignore non-leaf pmd_young for force_scan=true

[Yuanchu Xie <yuanchu@google.com>]

On Mon, Apr 8, 2024 at 11:52 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Yuanchu Xie <yuanchu@google.com> writes:
>
> > When non-leaf pmd accessed bits are available, MGLRU page table walks
> > can clear the accessed bit and promptly ignore the accessed bit on the
> > pte because it's on a different node, so the walk does not update the
> > generation of said page. When the next scan comes around on the right
> > node, the non-leaf pmd accessed bit might remain cleared and the pte
> > accessed bits won't be checked. While this is sufficient for
> > reclaim-driven aging, where the goal is to select a reasonably cold
> > page, the access can be missed when aging proactively for measuring the
> > working set size of a node/memcg.
> >
> > Since force_scan disables various other optimizations, we check
> > force_scan to ignore the non-leaf pmd accessed bit.
> >
> > Signed-off-by: Yuanchu Xie <yuanchu@google.com>
> > ---
> >  mm/vmscan.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4f9c854ce6cc..1a7c7d537db6 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3522,7 +3522,7 @@ static void walk_pmd_range(pud_t *pud, unsigned long start, unsigned long end,
> >
> >               walk->mm_stats[MM_NONLEAF_TOTAL]++;
> >
> > -             if (should_clear_pmd_young()) {
> > +             if (!walk->force_scan && should_clear_pmd_young()) {
> >                       if (!pmd_young(val))
> >                               continue;
>
> Sorry, I don't understand why we need this.  If !pmd_young(val), we
> don't need to update the generation.  If pmd_young(val), the bloom
> filter will be ignored if force_scan == true.  Or do I miss something?
If !pmd_young(val), we still might need to update the generation.

The get_pfn_folio function returns NULL if the folio's nid != node
under scanning,
so the pte accessed bit does not get cleared and the generation is not updated.
Now the pmd_young flag of this pmd is cleared, and if none of the
pte's are accessed
before another round of scanning occurs on the folio's node, the pmd_young check
fails and the pte accessed bit is skipped.

This is fine for kswapd but can introduce inaccuracies when scanning
proactively for
workingset estimation.

Thanks,
Yuanchu