[PATCH v3 4/4] mm: remove unnecessary smp_wmb() in __SetPageUptodate()

[Yu Zhao]

smp_wmb()s added in the previous patch guarantee that the user data
appears before a page is exposed by set_pte_at(). So there is no
need for __SetPageUptodate() to have a built-in one.

There are total 13 __SetPageUptodate() for the non-hugetlb case. 12
of them reuse smp_wmb()s added in the previous patch.

The one in shmem_mfill_atomic_pte() doesn't need a explicit write
barrier because of the following shmem_add_to_page_cache().

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
 include/linux/page-flags.h |  6 +++++-
 kernel/events/uprobes.c    |  2 +-
 mm/huge_memory.c           | 11 +++--------
 mm/khugepaged.c            |  2 +-
 mm/memory.c                | 13 ++++---------
 mm/migrate.c               |  7 +------
 mm/swapfile.c              |  2 +-
 mm/userfaultfd.c           |  7 +------
 8 files changed, 17 insertions(+), 33 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f91cb8898ff0..2481f9ad5f5b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -508,10 +508,14 @@ static inline int PageUptodate(struct page *page)
 	return ret;
 }
 
+/*
+ * Only use this function when there is a following write barrier, e.g.,
+ * an explicit smp_wmb() and/or the page will be added to page or swap
+ * cache locked.
+ */
 static __always_inline void __SetPageUptodate(struct page *page)
 {
 	VM_BUG_ON_PAGE(PageTail(page), page);
-	smp_wmb();
 	__set_bit(PG_uptodate, &page->flags);
 }
 
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 7069785e2e52..6ceae92afcc0 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -194,7 +194,7 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
 
 	flush_cache_page(vma, addr, pte_pfn(*pvmw.pte));
 	ptep_clear_flush_notify(vma, addr, pvmw.pte);
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb();
 	set_pte_at_notify(mm, addr, pvmw.pte,
 			mk_pte(new_page, vma->vm_page_prot));
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 21d271a29d96..101e7bd61e8f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -580,11 +580,6 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 	}
 
 	clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
-	/*
-	 * The memory barrier inside __SetPageUptodate makes sure that
-	 * clear_huge_page writes become visible before the set_pmd_at()
-	 * write.
-	 */
 	__SetPageUptodate(page);
 
 	vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
@@ -616,7 +611,7 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
 		mem_cgroup_commit_charge(page, memcg, false, true);
 		lru_cache_add_active_or_unevictable(page, vma);
 		pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 		set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
 		add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
@@ -1278,7 +1273,7 @@ static vm_fault_t do_huge_pmd_wp_page_fallback(struct vm_fault *vmf,
 	}
 	kfree(pages);
 
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb(); /* make pte visible before pmd */
 
 	pmd_populate(vma->vm_mm, vmf->pmd, pgtable);
@@ -1427,7 +1422,7 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
 		page_add_new_anon_rmap(new_page, vma, haddr, true);
 		mem_cgroup_commit_charge(new_page, memcg, false, true);
 		lru_cache_add_active_or_unevictable(new_page, vma);
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 		set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
 		update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f2901edce6de..668918842712 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1074,7 +1074,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	count_memcg_events(memcg, THP_COLLAPSE_ALLOC, 1);
 	lru_cache_add_active_or_unevictable(new_page, vma);
 	pgtable_trans_huge_deposit(mm, pmd, pgtable);
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb();
 	set_pmd_at(mm, address, pmd, _pmd);
 	update_mmu_cache_pmd(vma, address, pmd);
diff --git a/mm/memory.c b/mm/memory.c
index 6dabbc3cd3b7..db001d919e60 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2367,7 +2367,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
 		 * mmu page tables (such as kvm shadow page tables), we want the
 		 * new page to be mapped directly into the secondary page table.
 		 */
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 		set_pte_at_notify(mm, vmf->address, vmf->pte, entry);
 		update_mmu_cache(vma, vmf->address, vmf->pte);
@@ -2887,7 +2887,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		page_add_new_anon_rmap(page, vma, vmf->address, false);
 		mem_cgroup_commit_charge(page, memcg, false, false);
 		lru_cache_add_active_or_unevictable(page, vma);
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 	} else {
 		do_page_add_anon_rmap(page, vma, vmf->address, exclusive);
@@ -3006,11 +3006,6 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 					false))
 		goto oom_free_page;
 
-	/*
-	 * The memory barrier inside __SetPageUptodate makes sure that
-	 * preceeding stores to the page contents become visible before
-	 * the set_pte_at() write.
-	 */
 	__SetPageUptodate(page);
 
 	entry = mk_pte(page, vma->vm_page_prot);
@@ -3038,7 +3033,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
 	page_add_new_anon_rmap(page, vma, vmf->address, false);
 	mem_cgroup_commit_charge(page, memcg, false, false);
 	lru_cache_add_active_or_unevictable(page, vma);
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb();
 setpte:
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, entry);
@@ -3303,7 +3298,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
 		page_add_new_anon_rmap(page, vma, vmf->address, false);
 		mem_cgroup_commit_charge(page, memcg, false, false);
 		lru_cache_add_active_or_unevictable(page, vma);
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 	} else {
 		inc_mm_counter_fast(vma->vm_mm, mm_counter_file(page));
diff --git a/mm/migrate.c b/mm/migrate.c
index 943d147ecc3e..dc0ab9fbe36e 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2729,11 +2729,6 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
 	if (mem_cgroup_try_charge(page, vma->vm_mm, GFP_KERNEL, &memcg, false))
 		goto abort;
 
-	/*
-	 * The memory barrier inside __SetPageUptodate makes sure that
-	 * preceding stores to the page contents become visible before
-	 * the set_pte_at() write.
-	 */
 	__SetPageUptodate(page);
 
 	if (is_zone_device_page(page)) {
@@ -2783,7 +2778,7 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
 		lru_cache_add_active_or_unevictable(page, vma);
 	get_page(page);
 
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb();
 	if (flush) {
 		flush_cache_page(vma, addr, pte_pfn(*ptep));
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5c5547053ee0..dc9f1b1ba1a6 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1887,7 +1887,7 @@ static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
 		page_add_new_anon_rmap(page, vma, addr, false);
 		mem_cgroup_commit_charge(page, memcg, false, false);
 		lru_cache_add_active_or_unevictable(page, vma);
-		/* commit non-atomic ops before exposing to fast gup */
+		/* commit non-atomic ops and user data */
 		smp_wmb();
 	}
 	set_pte_at(vma->vm_mm, addr, pte,
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 4f92913242a1..34083680869e 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -58,11 +58,6 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 		*pagep = NULL;
 	}
 
-	/*
-	 * The memory barrier inside __SetPageUptodate makes sure that
-	 * preceeding stores to the page contents become visible before
-	 * the set_pte_at() write.
-	 */
 	__SetPageUptodate(page);
 
 	ret = -ENOMEM;
@@ -92,7 +87,7 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
 	mem_cgroup_commit_charge(page, memcg, false, false);
 	lru_cache_add_active_or_unevictable(page, dst_vma);
 
-	/* commit non-atomic ops before exposing to fast gup */
+	/* commit non-atomic ops and user data */
 	smp_wmb();
 	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
 
-- 
2.23.0.351.gc4317032e6-goog