FAILED: patch "[PATCH] mm/gup: fix always draining LRU caches in" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 078e1a0fc41a42baaf113383b52dab8874c0b967
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026090339-crisped-bouncy-5af4@gregkh' --subject-prefix 'PATCH 6.1.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 078e1a0fc41a42baaf113383b52dab8874c0b967 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <[email protected]>
Date: Fri, 31 Jul 2026 22:27:53 +0200
Subject: [PATCH] mm/gup: fix always draining LRU caches in
collect_longterm_unpinnable_folios()

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 ever have to drain.

index 8ea3de60e82d..bce275c7dbb6 100644
@@ -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;
 		}

From: "David Hildenbrand (Arm)" <[email protected]>

[ Upstream commit 078e1a0fc41a42baaf113383b52dab8874c0b967 ]

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 ever have to drain.

---
 mm/gup.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

index 72e01b8311d7c..bed94c3d6e82b 100644
@@ -1972,6 +1972,8 @@ static unsigned long collect_longterm_unpinnable_pages(
 
 	for (i = 0; i < nr_pages; i++) {
 		struct folio *folio = page_folio(pages[i]);
+		const int pin_refs = folio_test_large(folio) ?
+			1 : GUP_PIN_COUNTING_BIAS;
 
 		if (folio == prev_folio)
 			continue;

@@ -1990,15 +1992,20 @@ static unsigned long collect_longterm_unpinnable_pages(
 			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;
 		}

2.53.0