khugepaged: reuse the smp_wmb() inside __SetPageUptodate()

smp_wmb() is needed to avoid the copy_huge_page writes to become visible
after the set_pmd_at() write here.  But we can reuse the smp_wmb() inside
__SetPageUptodate() to remove this redundant one.

Link: https://lkml.kernel.org/r/20210306032947.35921-3-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/khugepaged.c

@@ -1183,19 +1183,18 @@ static void collapse_huge_page(struct mm_struct *mm,
 	__collapse_huge_page_copy(pte, new_page, vma, address, pte_ptl,
 			&compound_pagelist);
 	pte_unmap(pte);
+	/*
+	 * spin_lock() below is not the equivalent of smp_wmb(), but
+	 * the smp_wmb() inside __SetPageUptodate() can be reused to
+	 * avoid the copy_huge_page writes to become visible after
+	 * the set_pmd_at() write.
+	 */
 	__SetPageUptodate(new_page);
 	pgtable = pmd_pgtable(_pmd);
 
 	_pmd = mk_huge_pmd(new_page, vma->vm_page_prot);
 	_pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
 
-	/*
-	 * spin_lock() below is not the equivalent of smp_wmb(), so
-	 * this is needed to avoid the copy_huge_page writes to become
-	 * visible after the set_pmd_at() write.
-	 */
-	smp_wmb();
-
 	spin_lock(pmd_ptl);
 	BUG_ON(!pmd_none(*pmd));
 	page_add_new_anon_rmap(new_page, vma, address, true);