Subject: [PATCH v3 1/2] secretmem/gup: don't check if page is secretmem without reference

From: Mike Rapoport <[email-masked]>
Date: Tue, 20 Apr 2021 18:00:48 +0300

From: Mike Rapoport <[email-masked]>

The check in gup_pte_range() whether a page belongs to a secretmem mapping
is performed before grabbing the page reference.

To avoid potential race move the check after try_grab_compound_head().

Signed-off-by: Mike Rapoport <[email-masked]>
---
 mm/gup.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index c3a17b189064..6515f82b0f32 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2080,13 +2080,15 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
 
-		if (page_is_secretmem(page))
-			goto pte_unmap;
-
 		head = try_grab_compound_head(page, 1, flags);
 		if (!head)
 			goto pte_unmap;
 
+		if (unlikely(page_is_secretmem(page))) {
+			put_compound_head(head, 1, flags);
+			goto pte_unmap;
+		}
+
 		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
 			put_compound_head(head, 1, flags);
 			goto pte_unmap;
-- 
2.28.0

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