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

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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>
This commit is contained in:
John Hubbard 2024-02-27 19:41:51 -08:00 committed by Andrew Morton
parent c8b3600312
commit 6c1b748ebf
1 changed files with 6 additions and 6 deletions
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12
mm/memory.c
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@ -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);
/*