[PATCH 03/15] mm/hugetlb: preserve reservations when shrinking on HIGHMEM

["David Hildenbrand (Arm)" <david@kernel.org>]

try_to_free_low() drops the hugetlb_lock while freeing lowmem folios.
set_max_huge_pages() then resumes freeing folios using the
min_count computed before that unlock. Reservations created in between
can consequently lose their backing.

Collect lowmem folios on the caller's existing page_list instead. Keep
the hugetlb_lock held through both selection passes, and free the combined
list at the caller's existing unlock point.

Likely not stable-worthy.

Fixes: 1121828a0c21 ("hugetlb: call update_and_free_page without hugetlb_lock")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/hugetlb.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3a94ea1c9457..a78b2487c20c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3708,39 +3708,31 @@ static void __init report_hugepages(void)
 
 #ifdef CONFIG_HIGHMEM
 static void try_to_free_low(struct hstate *h, unsigned long count,
-						nodemask_t *nodes_allowed)
+		nodemask_t *nodes_allowed, struct list_head *page_list)
 {
 	int i;
-	LIST_HEAD(page_list);
 
 	lockdep_assert_held(&hugetlb_lock);
 	if (hstate_is_gigantic(h))
 		return;
 
-	/*
-	 * Collect pages to be freed on a list, and free after dropping lock
-	 */
+	/* Collect lowmem pages to free after dropping the lock. */
 	for_each_node_mask(i, *nodes_allowed) {
 		struct folio *folio, *next;
 		struct list_head *freel = &h->hugepage_freelists[i];
 		list_for_each_entry_safe(folio, next, freel, lru) {
 			if (count >= persistent_huge_pages(h))
-				goto out;
+				return;
 			if (folio_test_highmem(folio))
 				continue;
 			remove_hugetlb_folio(h, folio, false);
-			list_add(&folio->lru, &page_list);
+			list_add(&folio->lru, page_list);
 		}
 	}
-
-out:
-	spin_unlock_irq(&hugetlb_lock);
-	update_and_free_pages_bulk(h, &page_list);
-	spin_lock_irq(&hugetlb_lock);
 }
 #else
 static inline void try_to_free_low(struct hstate *h, unsigned long count,
-						nodemask_t *nodes_allowed)
+		nodemask_t *nodes_allowed, struct list_head *page_list)
 {
 }
 #endif
@@ -3935,7 +3927,7 @@ static int set_max_huge_pages(struct hstate *h, unsigned long count, int nid,
 	}
 	min_count = h->resv_huge_pages + persistent_huge_pages(h) - persistent_free_count;
 	min_count = max(count, min_count);
-	try_to_free_low(h, min_count, nodes_allowed);
+	try_to_free_low(h, min_count, nodes_allowed, &page_list);
 
 	/*
 	 * Collect pages to be removed on list without dropping lock

-- 
2.43.0