mm/memory.c: do_numa_page(): remove a redundant page table read

do_numa_page() is reading from the same page table entry, twice, while
holding the page table lock: once while checking that the pte hasn't
changed, and again in order to modify the pte.

Instead, just read the pte once, and save it in the same old_pte variable
that already exists.  This has no effect on behavior, other than to
provide a tiny potential improvement to performance, by avoiding the
redundant memory read (which the compiler cannot elide, due to
READ_ONCE()).

Also improve the associated comments nearby.

Link: https://lkml.kernel.org/r/20240228034151.459370-1-jhubbard@nvidia.com
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory.c b/mm/memory.c
index 75e0abda86427c918e6e7c7c5f3a33104d49d0eb..230bcd12a3362dc06d7ca1d2dbf060e10ca10c56 100644 (file)
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5127,18 +5127,18 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
        int flags = 0;
 
        /*
-        * The "pte" at this point cannot be used safely without
-        * validation through pte_unmap_same(). It's of NUMA type but
-        * the pfn may be screwed if the read is non atomic.
+        * The pte cannot be used safely until we verify, while holding the page
+        * table lock, that its contents have not changed during fault handling.
         */
        spin_lock(vmf->ptl);
-       if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) {
+       /* Read the live PTE from the page tables: */
+       old_pte = ptep_get(vmf->pte);
+
+       if (unlikely(!pte_same(old_pte, vmf->orig_pte))) {
                pte_unmap_unlock(vmf->pte, vmf->ptl);
                goto out;
        }
 
-       /* Get the normal PTE  */
-       old_pte = ptep_get(vmf->pte);
        pte = pte_modify(old_pte, vma->vm_page_prot);
 
        /*