Re: [PATCH v2 1/2] mm/huge_memory.c: skip zero-page remapping when underused THP shrinker is disabled

[Nico Pache]

On 10/25/26 10:52 PM, Barry Song wrote:
> On Mon, Oct 26, 2026 at 5:48 AM Nico Pache <nico.pache@xxxxxxxxx> wrote:
> > On 10/25/26 2:28 PM, Barry Song wrote:
> > > On Mon, Oct 26, 2026 at 1:40 AM Nico Pache (Red Hat)
> > > <nico.pache@xxxxxxxxx> wrote:
> > > > The zero-page remapping in __folio_split() was added to support the
> > > > underused THP shrinker. When the shrinker is disabled via
> > > > shrink_underused, gate TTU_USE_SHARED_ZEROPAGE on split_underused_thp
> > > > to avoid scanning subpages for zero content unnecessarily.
> > > >
> > > > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> > > > Signed-off-by: Nico Pache <nico.pache@xxxxxxxxx>
> > >
> > > is b1f202060afe a correct fixes tag? as
> > > commit b1f202060afe has no even "split_underused_thp" there.
> >
> > I believe so. IIUC these two series were merged together. While
> > 81d3ff3c6f7 introduces the toggle, b1f202060afe is where the behavior
> > started.
> >
> > I guess you could argue that the Fixes should include both?
> >
> > Not entirely sure!
>
> Thanks for the clarification. I guess it's fine.
>
> BTW, with the code below, we no longer add fully mapped
> folios to deferred_split_folio. So the scanning code only
> handles partially mapped folios to set up zero-page mappings
> for their subpages.
>
> I'm still not quite sure whether this is really harmful, though.
> Am I missing something, or was there some discussion I missed?

The issue is not in the actual shrinker code or the deferred split list.

The issue is that any split leads to this zero-page remapping 
indiscriminate of the actual toggle. The same way this toggle is gating 
things going on the deferred split list, we should be gating the actual 
zero-page remapping too.

There were two pieces to the underutilized shrinker, first we check if a 
THP is underutilized, if so we split, and the split+remapping handles 
the freeing of that memory. However, functionally this also made ANY 
split do this zero page remapping, not just those coming from the shrinker.

-- Nico

> void deferred_split_folio(struct folio *folio, bool partially_mapped)
> {
>          ...
>          /*
>           * Order 1 folios have no space for a deferred list, but we also
>           * won't waste much memory by not adding them to the deferred list.
>           */
>          if (folio_order(folio) <= 1)
>                  return;
>
>          if (!partially_mapped && !split_underused_thp)
>                  return;
> }