[PATCH v6 07/26] mm: Preserve the PG_arch_* flags in __split_huge_page_tail()

[Catalin Marinas]

When a huge page is split into normal pages, part of the head page flags
are transferred to the tail pages. However, the PG_arch_* flags are not
part of the preserved set.

PG_arch_1 is currently used by the arch code to handle cache maintenance
for user space (either for I-D cache coherency or for D-cache aliases
consistent with the kernel mapping). Since splitting a huge page does
not change the physical or virtual address of a mapping, additional
cache maintenance for the tail pages is unnecessary. Preserving the
PG_arch_1 flag from the head page in the tail pages would not break the
current use-cases.

PG_arch_2 is currently used for arm64 MTE support to mark pages that
have valid tags. The absence of such flag causes the arm64 set_pte_at()
to clear the tags in order to avoid stale tags exposed to user or the
swapping out hooks to ignore the tags. Not preserving PG_arch_2 on huge
page splitting leads to tag corruption in the tail pages.

To avoid the above and for consistency between the two PG_arch_* flags,
preserve both PG_arch_1 and PG_arch_2 in __split_huge_page_tail().

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
---

Notes:
    New in v6.

 mm/huge_memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78c84bee7e29..22b3236a6dd8 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2364,6 +2364,10 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_workingset) |
 			 (1L << PG_locked) |
 			 (1L << PG_unevictable) |
+			 (1L << PG_arch_1) |
+#ifdef CONFIG_64BIT
+			 (1L << PG_arch_2) |
+#endif
 			 (1L << PG_dirty)));
 
 	/* ->mapping in first tail page is compound_mapcount */