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From: John Hubbard <jhubbard@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
CC: LKML <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>, John Hubbard
	<jhubbard@nvidia.com>, Ryan Roberts <ryan.roberts@arm.com>, David Hildenbrand
	<david@redhat.com>
Subject: [PATCH] mm/memory.c: do_numa_page(): remove a redundant page table read
Date: Tue, 27 Feb 2024 19:41:51 -0800
Message-ID: <20240228034151.459370-1-jhubbard@nvidia.com>
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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.

Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/memory.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 0bfc8b007c01..df0711982901 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4928,18 +4928,18 @@ 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);
 
 	/*
-- 
2.44.0