Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem without reference

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On 20.04.21 15:16, Mike Rapoport wrote:
From: Mike Rapoport <rppt@xxxxxxxxxxxxx>

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 <rppt@xxxxxxxxxxxxx>
---
  mm/gup.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index c3a17b189064..4b58c016e949 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2080,13 +2080,13 @@ 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 (page_is_secretmem(page))
+			goto pte_unmap;
+

Looking at the hunk below, I wonder if you're missing a put_compound_head().

(also, I'd do if unlikely(page_is_secretmem()) but that's a different discussion)

  		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
  			put_compound_head(head, 1, flags);
  			goto pte_unmap;



--
Thanks,

David / dhildenb





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