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

[Barry Song]

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? We might end up doing some extra draining in that case,
but that's safe—any value greater than 1 might not be. We may just
drain more than necessary?

Best Regards
Barry