[PATCH v2] mm: release private data before split THP

[Yin Fengwei <fengwei.yin@intel.com>]

If there is private data attached to THP, the refcount of
THP will be increased and block the THP split. Release
private data attached to THP before split it to increase
the chance of splitting THP successfully.

There was a memory failure issue hit during HW error
injection testing with 5.18 kernel + xfs as rootfs. Test
got killed and system reboot was required to re-run the
test.

The issue was tracked down to THP split failure caused the
memory failure not being handled. The page dump showed:

[ 1785.433075] page:0000000025f9530b refcount:18 mapcount:0 mapping:000000008162eea7 index:0xa10 pfn:0x2f0200
[ 1785.443954] head:0000000025f9530b order:4 compound_mapcount:0 compound_pincount:0
[ 1785.452408] memcg:ff4247f2d28e9000
[ 1785.456304] aops:xfs_address_space_operations ino:8555182 dentry name:"baseos-filenames.solvx"
[ 1785.466612] flags: 0x1000000000012036(referenced|uptodate|lru|active|private|head|node=0|zone=2)
[ 1785.476514] raw: 1000000000012036 ffb9460f8bc07c08 ffb9460f8bc08408 ff4247f22e6299f8
[ 1785.485268] raw: 0000000000000a10 ff4247f194ade900 00000012ffffffff ff4247f2d28e9000

It was like the error was injected to a large folio for xfs
with private data attached.

With private data released before split THP, the test case
could be run successfully many times without reboot system.

Co-developed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
---
Changelog from v1:
 - Move private release to split_huge_page_to_list
   to cover wider path per Yang's comment
 - Update to commit message

Changelog from RFC:
 - Use new folio API per Mathhew Wilcox's suggestion
 - Add one line comment before re-get folio of page per
   Miaohe's comment
 - Remove RFC tag
 - Add Co-developed-by of Qiuxu who did a lot of debugging
   work to locate where the real issue is

 mm/huge_memory.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 15965084816d..edcbc6c2bb3f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2590,6 +2590,12 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 			goto out;
 		}
 
+		if (folio_test_private(folio) &&
+				!filemap_release_folio(folio, GFP_KERNEL)) {
+			ret = -EBUSY;
+			goto out;
+		}
+
 		xas_split_alloc(&xas, head, compound_order(head),
 				mapping_gfp_mask(mapping) & GFP_RECLAIM_MASK);
 		if (xas_error(&xas)) {

base-commit: 31be1d0fbd950395701d9fd47d8fb1f99c996f61
-- 
2.25.1