Commit e2491c31 authored by John Hubbard's avatar John Hubbard Committed by Kefeng Wang
Browse files

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

mainline inclusion
from mainline-v6.9-rc1
commit 6c1b748ebf27befffec83b77ca1960bf70ed6ac9
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I9S4Z4
CVE: NA

-------------------------------------------------

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: default avatarJohn Hubbard <jhubbard@nvidia.com>
Reviewed-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarRyan Roberts <ryan.roberts@arm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 6c1b748ebf27befffec83b77ca1960bf70ed6ac9)
Signed-off-by: default avatarKefeng Wang <wangkefeng.wang@huawei.com>
parent 8821af3b

mm/memory.c

+6 −6
Original line number Diff line number Diff line
@@ -5132,18 +5132,18 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) 
	int flags = 0, nr_pages;



	/*

	 * 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);



	/*