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