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

[Barry Song]

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?

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;
}