Re: [PATCH 1/1] mm: avoid processing mlocked THPs in deferred split shrinker

[Lance Yang <lance.yang@linux.dev>]

On 2025/9/8 15:38, David Hildenbrand wrote:
> On 08.09.25 06:07, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
> 
> Subject should likely be more specific:
> 
> mm: skip mlocked THPs that are underused early in deferred_split_scan()

Right, that's a much better and more precise subject. Thanks!

> 
>>
>> When a new THP is faulted in or collapsed, it is unconditionally added to
>> the deferred split queue. If this THP is subsequently mlocked, it remains
>> on the queue but is removed from the LRU and marked unevictable.
>>
>> During memory reclaim, deferred_split_scan() will still pick up this 
>> large
>> folio. Because it's not partially mapped, it will proceed to call
>> thp_underused() and then attempt to split_folio() to free all zero-filled
>> subpages.
>>
>> This is a pointless waste of CPU cycles. The folio is mlocked and
>> unevictable, so any attempt to reclaim memory from it via splitting is
>> doomed to fail.
> 
> I think the whole description is a bit misleading: we're not reclaiming
> memory from fully-mapped THPs even when they are underused, because it
> could violate mlock() semantics where we don't want a page fault+memory 
> allocation on next access.
> 
> So something like the following might be clearer.
> 
> "When we stumble over a fully-mapped THP in the deferred shrinker, it 
> does not make sense trying to detect whether it is underused, because 
> try_to_map_unused_to_zeropage(), called while splitting the folio, will 
> not actually replace any zero-ed pages by the shared zeropage.
> 
> Splitting the folio in that case does not make any sense, so let's not 
> even scan if the folio is underused.
> "

Nice, that makes it much clearer. My understanding was indeed imprecise.

> 
> 
> 
> If I run my reproducer from [1] and mlock() the pages just after 
> allocating them, then I essentially get
> 
> AnonHugePages:   1048576 kB
> 
> converted to
> 
> Anonymous:       1048580 kB
> 
> Which makes sense (no memory optimized out) as discussed above.

Yes, my reproducer also shows exactly that. It's clear a lot of work is
done but no memory is actually optimized out ;)

> 
> 
> [1] https://lkml.kernel.org/r/20250905141137.3529867-1-david@redhat.com
> 
>>
>> So, let's add an early folio_test_mlocked() check to skip this case.
>>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/huge_memory.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 77f0c3417973..d2e84015d6b4 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -4183,6 +4183,9 @@ static unsigned long deferred_split_scan(struct 
>> shrinker *shrink,
>>           bool underused = false;
>>           if (!folio_test_partially_mapped(folio)) {
>> +            /* An mlocked folio is not a candidate for the shrinker. */
> 
> /*
>   * See try_to_map_unused_to_zeropage(): we cannot optimize zero-filled
>   * pages after splitting an mlocked folio.
>   */

Got it. I'll update the changelog and this comment as suggested.

> 
>> +            if (folio_test_mlocked(folio))
>> +                goto next;
>>               underused = thp_underused(folio);
>>               if (!underused)
>>                   goto next;
> 
>

Cheers,
Lance