Re: [PATCH v1 2/2] mm/memory: add anonymous mTHP folios to deferred split list

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/4/26 16:03, Johannes Weiner wrote:
> On Tue, Aug 04, 2026 at 10:03:14AM +0800, Barry Song wrote:
>> On Tue, Aug 4, 2026 at 8:48 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>>>
>> [...]
>>>
>>> I am very confused. Did you not see my proposal above?
>>>
>>> Why not this?
>>
>> Hi Johannes,
>>
>> For arm64, if the base page size is 64KB, a PMD would be 512MB,
>> and PMD-1 would be 256MB. Usama mentioned 2MB, which is just
>> order-5, not PMD-1 on arm64.
>>
>> BTW, I assume khugepaged_max_ptes_none is intended for collapse,
>> not splitting. I am a bit concerned that reusing it for this
>> purpose would be quite disruptive.
> 
> It already is:
> 
> static bool thp_underused(struct folio *folio)
> {
> 	int num_zero_pages = 0, num_filled_pages = 0;
> 	int i;
> 
> 	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> 		return false;
> 
> 	if (folio_contain_hwpoisoned_page(folio))
> 		return false;
> 
> 	for (i = 0; i < folio_nr_pages(folio); i++) {
> 		if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {
> 			if (++num_zero_pages > khugepaged_max_ptes_none)
> 				return true;
> 		} else {
> 			/*
> 			 * Another path for early exit once the number
> 			 * of non-zero filled pages exceeds threshold.
> 			 */
> 			if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none)
> 				return false;
> 		}
> 	}
> 	return false;
> }
> 
> That's ABI and setups are relying on it.
> 
> All I'm proposing is to only queue pages that the shrinker would
> actually split under currently existing rules. That's a mostly
> transparent optimization, not a new policy.

I yet have to reply to some stuff here (sorry, it's a mess :( ), but one thing
that annoyed me right from the start is that we have:

static bool split_underused_thp = true;

But the default khugepaged settings actually don't result in any splits.

I think it would already be a big improvement to not default to adding all pages
in default setups.

When we last discussed changing that, one concern raised in the past was that
changing the khugepaged setting later would then not be able to reclaim pages
allocated up to that point.

When someone actually changes khugepaged settings such that the underused
shrinker would get active, could we walk the LRU and add all eligible anon THPs
to the underused shrinker? We could remember "already scanned for underused" per
THP, but maybe we could avoid that initially.

-- 
Cheers,

David