Re: [RFC] mm: restrict zero-page remapping to underused THP splits

[David Hildenbrand (Arm)]

On 5/14/26 10:11, Lance Yang wrote:
> 
> On Tue, May 12, 2026 at 09:02:38PM +0200, David Hildenbrand (Arm) wrote:
>> On 5/12/26 20:36, Nico Pache wrote:
>>> On Tue, May 12, 2026 at 1:05 AM David Hildenbrand (Arm)
>>> <david@xxxxxxxxxx> wrote:
>>>
>>> I meant before b1f202060afe ("mm: remap unused subpages to shared
>>> zeropage when splitting isolated thp"). Sorry, I should have been more
>>> specific.
>>>
>>> As in before the underutilized shrinker (and commit b1f202060afe), we
>>> had the exact problem you describe above and no way to handle it.
>>> Correct?
>>
>> Right. That's why the underused shrinker was added, to directly free pages that
>> have been over-allocated with a THP, but are actually never "used" (remain zero).
>>
>>>
>>> I guess at reclaim THPs would be split and we would swap out a zero filled page?
>> We don't necessarily split during swapout, we try to swap out the THP if
>> supported. But yes, that can happen.
>>
>> I'm more thinking about other reasons to split a THP (e.g., migration, partial
>> MADV_PAGEOUT/MADV_COLD/MADV_FREE), where the behavior would be changed.
> 
> Yeah. Today any successful anon split gets TTU_USE_SHARED_ZEROPAGE. Would
> it be better to make KSM opt out, so other split paths keep the behavior
> they have today?
> 
> That way we do not have to worry about changing anything else at the
> same time :D

This is (1) here:

https://lore.kernel.org/all/04ea0e68-de56-49c4-8c9f-1734139d5e7f@xxxxxxxxxx/

There will still be interaction between both mechanisms. So I'd prefer that in
VM_MERGEABLE sections with KSM enabled, only KSM would take care of deduplicating.

-- 
Cheers,

David