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

[Nico Pache <nico.pache@linux.dev>]

On 10/25/26 10:52 PM, Barry Song wrote:
> On Mon, Oct 26, 2026 at 5:48 AM Nico Pache <nico.pache@linux.dev> 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@linux.dev> 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@kernel.org>
>>>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
>>>> Signed-off-by: Nico Pache <nico.pache@linux.dev>
>>>
>>> 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;
> }