Re: [PATCH v2 05/14] mm: khugepaged: __collapse_huge_page_swapin() use pte_offset_map_ro_nolock()

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On 22.08.24 09:13, Qi Zheng wrote:
In __collapse_huge_page_swapin(), we just use the ptl for pte_same() check
in do_swap_page(). In other places, we directly use pte_offset_map_lock(),
so convert it to using pte_offset_map_ro_nolock().

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
  mm/khugepaged.c | 6 +++++-
  1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4a83c40d90538..53bfa7f4b7f82 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1011,7 +1011,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
  		};
if (!pte++) {
-			pte = pte_offset_map_nolock(mm, pmd, address, &ptl);
+			/*
+			 * Here the ptl is only used to check pte_same() in
+			 * do_swap_page(), so readonly version is enough.
+			 */
+			pte = pte_offset_map_ro_nolock(mm, pmd, address, &ptl);
  			if (!pte) {
  				mmap_read_unlock(mm);
  				result = SCAN_PMD_NULL;

Suboptimal that the pteval comparison + unmap is buried in do_swap_page(). Moving that to the caller is also not significantly better ...

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb





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