From 6c1b748ebf27befffec83b77ca1960bf70ed6ac9 Mon Sep 17 00:00:00 2001 From: John Hubbard Date: Tue, 27 Feb 2024 19:41:51 -0800 Subject: [PATCH] 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 Reviewed-by: David Hildenbrand Reviewed-by: Ryan Roberts Signed-off-by: Andrew Morton --- mm/memory.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 75e0abda8642..230bcd12a336 100644 --- 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); /*