linux-kernel - [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read

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Date:   Wed, 9 Jun 2021 23:38:11 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Wang Yugui <wangyugui@...-tech.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>, Zi Yan <ziy@...dia.com>,
        Peter Xu <peterx@...hat.com>, Will Deacon <will@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read_atomic()

page_vma_mapped_walk() cleanup: use pmd_read_atomic() with barrier()
instead of READ_ONCE() for pmde: some architectures (e.g. i386 with PAE)
have a multi-word pmd entry, for which READ_ONCE() is not good enough.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
Cc: <stable@...r.kernel.org>
---
 mm/page_vma_mapped.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 7c0504641fb8..973c3c4e72cc 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -182,13 +182,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 	pud = pud_offset(p4d, pvmw->address);
 	if (!pud_present(*pud))
 		return false;
+
 	pvmw->pmd = pmd_offset(pud, pvmw->address);
 	/*
 	 * Make sure the pmd value isn't cached in a register by the
 	 * compiler and used as a stale value after we've observed a
 	 * subsequent update.
 	 */
-	pmde = READ_ONCE(*pvmw->pmd);
+	pmde = pmd_read_atomic(pvmw->pmd);
+	barrier();
+
 	if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
 		pvmw->ptl = pmd_lock(mm, pvmw->pmd);
 		if (likely(pmd_trans_huge(*pvmw->pmd))) {
-- 
2.26.2