[RFC PATCH 3/3] mm/memory-failure: handle min_order_for_split() error code properly

[Zi Yan]

min_order_for_split() returns -EBUSY when the folio is truncated and cannot
be split. In commit 77008e1b2ef7 ("mm/huge_memory: do not change
split_huge_page*() target order silently"), memory_failure() does not
handle it and pass -EBUSY to try_to_split_thp_page() directly.
try_to_split_thp_page() returns -EINVAL since -EBUSY becomes 0xfffffff0 as
new_order is unsigned int in __folio_split() and this large new_order is
rejected as an invalid input. The code does not cause a bug.
soft_offline_in_use_page() also uses min_order_for_split() but it always
passes 0 as new_order for split.

Handle it properly by checking min_order_for_split() return value and not
calling try_to_split_thp_page() if the value is negative. Add a comment
in soft_offline_in_use_page() to clarify the possible negative new_order
value.

Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/memory-failure.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 7f908ad795ad..86582f030159 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -2437,8 +2437,11 @@ int memory_failure(unsigned long pfn, int flags)
 		 * or unhandlable page.  The refcount is bumped iff the
 		 * page is a valid handlable page.
 		 */
-		folio_set_has_hwpoisoned(folio);
-		err = try_to_split_thp_page(p, new_order, /* release= */ false);
+		if (new_order >= 0) {
+			folio_set_has_hwpoisoned(folio);
+			err = try_to_split_thp_page(p, new_order, /* release= */ false);
+		} else
+			err = new_order;
 		/*
 		 * If splitting a folio to order-0 fails, kill the process.
 		 * Split the folio regardless to minimize unusable pages.
@@ -2779,6 +2782,7 @@ static int soft_offline_in_use_page(struct page *page)
 		/*
 		 * If new_order (target split order) is not 0, do not split the
 		 * folio at all to retain the still accessible large folio.
+		 * new_order can be -EBUSY, meaning the folio cannot be split.
 		 * NOTE: if minimizing the number of soft offline pages is
 		 * preferred, split it to non-zero new_order like it is done in
 		 * memory_failure().
-- 
2.51.0