Re: [RFC PATCH v3 0/4] mm: enable lru cache for smaller large folios

[David Hildenbrand (Arm)]

On 8/20/26 22:18, Barry Song wrote:
> On Fri, Aug 21, 2026 at 2:23 AM David Hildenbrand (Arm)
> <david@xxxxxxxxxx> wrote:
>>
>> On 8/19/26 00:59, Barry Song (Xiaomi) wrote:
>>> This patchset enables the per-CPU LRU cache for large folios with fewer
>>> than `FOLIO_BATCH_SIZE` (31) pages. It also limits each per-CPU LRU cache
>>> to at most `FOLIO_BATCH_SIZE` pages to avoid negatively affecting
>>> accounting and memory reclamation pressure.
>>>
>>> This is particularly beneficial on systems that use relatively small
>>> large folios. For larger folios, the benefit is likely to be smaller
>>> because far fewer folios are expected to contend for the LRU cache.
>>
>> As raised, there is this problem with collect_longterm_unpinnable_folios()
>>
>> (see
>> https://lore.kernel.org/r/20260806-lru_cache_drain_for_folio-v1-1-c6287d295e99@xxxxxxxxxx
>> )
>>
>> whereby we don't know how many refs we actually hold. Certainly not 1.
>>
>> We might have to wait for Hugh's cleanup to handle that cleanly (and avoid all
>> the other LRU cache draining).
> 
> Hi David,
> Thanks for raising this.
> Yes, I saw your comment and took a closer look at it. I think the
> best approach for now is to leave that part untouched until Hugh's
> patch lands? 

I think we should not consider your patch set until Hugh either resolved it or
we have a better way to handle draining. Building on top of this for large
folios now just gets painful.

> We might end up doing some extra draining in that case,
> but that's safe—any value greater than 1 might not be. 

We'll unconditionally drain all LRU caches, which is precisely *not* what we
want to do in the first place. :)

-- 
Cheers,

David