linux-kernel - [PATCH v2] mm/migrate: fix shmem xarray update during migration

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Message-ID: <20250228174953.2222831-1-ziy@nvidia.com>
Date: Fri, 28 Feb 2025 12:49:53 -0500
From: Zi Yan <ziy@...dia.com>
To: Liu Shixin <liushixin2@...wei.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Barry Song <baohua@...nel.org>,
	David Hildenbrand <david@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Lance Yang <ioworker0@...il.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Matthew Wilcox <willy@...radead.org>,
	Hugh Dickins <hughd@...gle.com>,
	Charan Teja Kalla <quic_charante@...cinc.com>,
	linux-kernel@...r.kernel.org,
	Zi Yan <ziy@...dia.com>,
	Shivank Garg <shivankg@....com>
Subject: [PATCH v2] mm/migrate: fix shmem xarray update during migration

Pagecache uses multi-index entries for large folio, so does shmem. Only
swap cache still stores multiple entries for a single large folio.
Commit fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
fixed swap cache but got shmem wrong by storing multiple entries for
a large shmem folio. Fix it by storing a single entry for a shmem
folio.

Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Reported-by: Liu Shixin <liushixin2@...wei.com>
Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
Signed-off-by: Zi Yan <ziy@...dia.com>
Reviewed-by: Shivank Garg <shivankg@....com>
---
 mm/migrate.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 365c6daa8d1b..2c9669135a38 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -524,7 +524,11 @@ static int __folio_migrate_mapping(struct address_space *mapping,
 			folio_set_swapcache(newfolio);
 			newfolio->private = folio_get_private(folio);
 		}
-		entries = nr;
+		/* shmem uses high-order entry */
+		if (!folio_test_anon(folio))
+			entries = 1;
+		else
+			entries = nr;
 	} else {
 		VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
 		entries = 1;
-- 
2.47.2