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. Fixes: 98c6d259319e ("mm/gup: check ref_count instead of lru before migration") Fixes: a09a8a1fbb37 ("mm/gup: local lru_add_drain() to avoid lru_add_drain_all()") Cc: stable@vger.kernel.org Signed-off-by: David Hildenbrand (Arm) --- Found by code inspection. If someone has a testcase that can easily trigger this and result in migration problems, please test! But this change seems to be "obvious the right thing to do". --- mm/gup.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 99902c15703b0..41c3317e0f0f4 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2273,6 +2273,7 @@ static unsigned long collect_longterm_unpinnable_folios( for (folio = pofs_get_folio(pofs, i); folio; folio = pofs_next_folio(folio, pofs, &i)) { + const int pin_refs = folio_has_pincount(folio) ? 1 : GUP_PIN_COUNTING_BIAS; if (folio_is_longterm_pinnable(folio)) continue; @@ -2287,15 +2288,20 @@ static unsigned long collect_longterm_unpinnable_folios( continue; } + /* + * We drain not only to make the folio_isolate_lru() succeed, + * but also to remove any other folio references from LRU + * caches. + */ if (drained == 0 && folio_may_be_lru_cached(folio) && folio_ref_count(folio) != - folio_expected_ref_count(folio) + 1) { + folio_expected_ref_count(folio) + pin_refs) { lru_add_drain(); drained = 1; } if (drained == 1 && folio_may_be_lru_cached(folio) && folio_ref_count(folio) != - folio_expected_ref_count(folio) + 1) { + folio_expected_ref_count(folio) + pin_refs) { lru_add_drain_all(); drained = 2; } --- base-commit: e5492213654050379e78ec6f9acfd6c9fe00f334 change-id: 20260731-check_and_migrate_movable_folios-43500556943d -- Cheers, David