Re: [PATCH] mm/gup: fix always draining LRU caches in collect_longterm_unpinnable_folios()

[David Hildenbrand (Arm)]

On 7/31/26 22:27, David Hildenbrand (Arm) wrote:
> folio_may_be_lru_cached() is currently only true for small folios, and
> for small folios FOLL_PIN adds GUP_PIN_COUNTING_BIAS references instead
> of 1 in try_grab_folio()/try_grab_folio_fast().
> 
> Consequently, our
> 
> 	folio_ref_count(folio) != folio_expected_ref_count(folio) + 1
> 
> check in collect_longterm_unpinnable_folios() will currently always
> identify "reference mismatch" and first drain the local LRU cache to then
> drain the LRU cache on all CPUs, as collect_longterm_unpinnable_folios()
> is really called after pinning the folios with FOLL_PIN.
> 
> Add a comment because the current code is not quite intuitive: we used to
> drain only to make sure the folio_isolate_lru() would succeed. But then we
> also started draining to make later migration more reliable.
> 
> We'll refactor that code soon a bit, to also make it usable in other
> context where we really want to remove any references from LRU caches.
> 
> Let's add CC stable, because having an easy way for excessive LRU cache
> draining on all CPUs does not sound right. In common scenarios we
> don't expect to every have to drain.

Stumbling over this:

s/every/ever/

Thanks for the review Hugh!

-- 
Cheers,

David