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Date: Tue, 4 Jun 2024 13:47:38 +0800 (CST)
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From: Ran Xiaokai <ran.xiaokai@zte.com.cn>

When I did a large folios split test, a WARNING
"[ 5059.122759][  T166] Cannot split file folio to non-0 order"
was triggered. But my test cases are only for anonmous folios. 
while mapping_large_folio_support() is only reasonable for page
cache folios. 

In split_huge_page_to_list_to_order(), the folio passed to
mapping_large_folio_support() maybe anonmous folio. The
folio_test_anon() check is missing. So the split of the anonmous THP
is failed. This is also the same for shmem_mapping(). We'd better add
a check for both. But the shmem_mapping() in __split_huge_page() is
not involved, as for anonmous folios, the end parameter is set to -1, so
(head[i].index >= end) is always false. shmem_mapping() is not called.

Using /sys/kernel/debug/split_huge_pages to verify this, with this
patch, large anon THP is successfully split and the warning is ceased.

Signed-off-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Yang Yang <yang.yang29@zte.com.cn>
---
 mm/huge_memory.c | 38 ++++++++++++++++++++------------------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 317de2afd371..4c9c7e5ea20c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 	if (new_order >= folio_order(folio))
 		return -EINVAL;

-	/* Cannot split anonymous THP to order-1 */
-	if (new_order == 1 && folio_test_anon(folio)) {
-		VM_WARN_ONCE(1, "Cannot split to order-1 folio");
-		return -EINVAL;
-	}
-
 	if (new_order) {
 		/* Only swapping a whole PMD-mapped folio is supported */
 		if (folio_test_swapcache(folio))
 			return -EINVAL;
-		/* Split shmem folio to non-zero order not supported */
-		if (shmem_mapping(folio->mapping)) {
-			VM_WARN_ONCE(1,
-				"Cannot split shmem folio to non-0 order");
-			return -EINVAL;
-		}
-		/* No split if the file system does not support large folio */
-		if (!mapping_large_folio_support(folio->mapping)) {
-			VM_WARN_ONCE(1,
-				"Cannot split file folio to non-0 order");
-			return -EINVAL;
+
+		if (folio_test_anon(folio)) {
+			/* Cannot split anonymous THP to order-1 */
+			if (new_order == 1) {
+				VM_WARN_ONCE(1, "Cannot split to order-1 folio");
+				return -EINVAL;
+			}
+		} else {
+			/* Split shmem folio to non-zero order not supported */
+			if (shmem_mapping(folio->mapping)) {
+				VM_WARN_ONCE(1,
+					"Cannot split shmem folio to non-0 order");
+				return -EINVAL;
+			}
+			/* No split if the file system does not support large folio */
+			if (!mapping_large_folio_support(folio->mapping)) {
+				VM_WARN_ONCE(1,
+					"Cannot split file folio to non-0 order");
+				return -EINVAL;
+			}
 		}
 	}

-
 	is_hzp = is_huge_zero_folio(folio);
 	if (is_hzp) {
 		pr_warn_ratelimited("Called split_huge_page for huge zero page\n");
-- 
2.15.2