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

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

[David Hildenbrand (Arm)]

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) <david@kernel.org>
---
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

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

[Hugh Dickins]

On Fri, 31 Jul 2026, 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.
> 
> 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) <david@kernel.org>

Ughh! And I was so proud of it. Now I'm ashamed. It just never
occurred to me, that the unpinnable pages were pinned there.
Thanks for the fix (but please don't tell anyone ;)

Acked-by: Hugh Dickins <hughd@google.com>

> ---
> 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".

Sorry, I don't (as perhaps is too obvious). And I'm a wee bit afraid
that some of the problems which those changes happened to succeed in
fixing, might now reappear once the drains are finely targeted.

Am I arguing for a delay before advancing to stable?  Perhaps, but
I think not: it's probably best to get that working as intended,
then deal with any fallout if it arises: just be on guard.

> ---
>  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

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

[Andrew Morton]

On Sat, 1 Aug 2026 11:49:39 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:

> > 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".
> 
> Sorry, I don't (as perhaps is too obvious). And I'm a wee bit afraid
> that some of the problems which those changes happened to succeed in
> fixing, might now reappear once the drains are finely targeted.
> 
> Am I arguing for a delay before advancing to stable?  Perhaps, but
> I think not: it's probably best to get that working as intended,
> then deal with any fallout if it arises: just be on guard.

This patch is in mm-new, so it's still some months away from -stable
eligibility.