[PATCH v7 07/29] mm: Preserve the PG_arch_2 flag in __split_huge_page_tail()

[Catalin Marinas <catalin.marinas@arm.com>]

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_2 is used by the arm64 MTE support to mark pages that have valid
tags. The absence of such flag would cause 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.

Preserve the newly added PG_arch_2 flag in __split_huge_page_tail().

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

Notes:
    v7:
    - Only preserve PG_arch_2 in __split_huge_page_tail(). The PG_arch_1
      flag will be discussed separately as it may potentially impact s390
      and x86.
    
    New in v6.

 mm/huge_memory.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78c84bee7e29..21ca2fff6b19 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2364,6 +2364,9 @@ static void __split_huge_page_tail(struct page *head, int tail,
 			 (1L << PG_workingset) |
 			 (1L << PG_locked) |
 			 (1L << PG_unevictable) |
+#ifdef CONFIG_64BIT
+			 (1L << PG_arch_2) |
+#endif
 			 (1L << PG_dirty)));
 
 	/* ->mapping in first tail page is compound_mapcount */