FAILED: patch "[PATCH] khugepaged: retract_page_tables() remember to test exit" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 18e77600f7a1ed69f8ce46c9e11cad0985712dfa Mon Sep 17 00:00:00 2001
From: Hugh Dickins <[email protected]>
Date: Thu, 6 Aug 2020 23:26:22 -0700
Subject: [PATCH] khugepaged: retract_page_tables() remember to test exit

Only once have I seen this scenario (and forgot even to notice what forced the eventual crash): a sequence of "BUG: Bad page map" alerts from vm_normal_page(), from zap_pte_range() servicing exit_mmap(); pmd:00000000, pte values corresponding to data in physical page 0.

The pte mappings being zapped in this case were supposed to be from a huge page of ext4 text (but could as well have been shmem): my belief is that it was racing with collapse_file()'s retract_page_tables(), found *pmd pointing to a page table, locked it, but *pmd had become 0 by the time start_pte was decided.

In most cases, that possibility is excluded by holding mmap lock; but exit_mmap() proceeds without mmap lock. Most of what's run by khugepaged checks khugepaged_test_exit() after acquiring mmap lock:
khugepaged_collapse_pte_mapped_thps() and hugepage_vma_revalidate() do so, for example. But retract_page_tables() did not: fix that.

The fix is for retract_page_tables() to check khugepaged_test_exit(), after acquiring mmap lock, before doing anything to the page table. Getting the mmap lock serializes with __mmput(), which briefly takes and drops it in __khugepaged_exit(); then the khugepaged_test_exit() check on mm_users makes sure we don't touch the page table once exit_mmap() might reach it, since exit_mmap() will be proceeding without mmap lock, not expecting anyone to be racing with it.

index a9aca9b71d6f..ac04b332a373 100644
@@ -1532,6 +1532,7 @@ static int khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
 static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 {
 	struct vm_area_struct *vma;
+	struct mm_struct *mm;
 	unsigned long addr;
 	pmd_t *pmd, _pmd;
 

@@ -1560,7 +1561,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			continue;
 		if (vma->vm_end < addr + HPAGE_PMD_SIZE)
 			continue;
-		pmd = mm_find_pmd(vma->vm_mm, addr);
+		mm = vma->vm_mm;
+		pmd = mm_find_pmd(mm, addr);
 		if (!pmd)
 			continue;
 		/*

@@ -1570,17 +1572,19 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		 * mmap_lock while holding page lock. Fault path does it in
 		 * reverse order. Trylock is a way to avoid deadlock.
 		 */
-		if (mmap_write_trylock(vma->vm_mm)) {
-			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
-			/* assume page table is clear */
-			_pmd = pmdp_collapse_flush(vma, addr, pmd);
-			spin_unlock(ptl);
-			mmap_write_unlock(vma->vm_mm);
-			mm_dec_nr_ptes(vma->vm_mm);
-			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
+		if (mmap_write_trylock(mm)) {
+			if (!khugepaged_test_exit(mm)) {
+				spinlock_t *ptl = pmd_lock(mm, pmd);
+				/* assume page table is clear */
+				_pmd = pmdp_collapse_flush(vma, addr, pmd);
+				spin_unlock(ptl);
+				mm_dec_nr_ptes(mm);
+				pte_free(mm, pmd_pgtable(_pmd));
+			}
+			mmap_write_unlock(mm);
 		} else {
 			/* Try again later */
-			khugepaged_add_pte_mapped_thp(vma->vm_mm, addr);
+			khugepaged_add_pte_mapped_thp(mm, addr);
 		}
 	}
 	i_mmap_unlock_write(mapping);

------------------ rebased commit in Linus's tree ------------------

From 18e77600f7a1ed69f8ce46c9e11cad0985712dfa Mon Sep 17 00:00:00 2001
From: Hugh Dickins <[email protected]>
Date: Thu, 6 Aug 2020 23:26:22 -0700
Subject: [PATCH] khugepaged: retract_page_tables() remember to test exit

Only once have I seen this scenario (and forgot even to notice what forced the eventual crash): a sequence of "BUG: Bad page map" alerts from vm_normal_page(), from zap_pte_range() servicing exit_mmap(); pmd:00000000, pte values corresponding to data in physical page 0.

The pte mappings being zapped in this case were supposed to be from a huge page of ext4 text (but could as well have been shmem): my belief is that it was racing with collapse_file()'s retract_page_tables(), found *pmd pointing to a page table, locked it, but *pmd had become 0 by the time start_pte was decided.

In most cases, that possibility is excluded by holding mmap lock; but exit_mmap() proceeds without mmap lock. Most of what's run by khugepaged checks khugepaged_test_exit() after acquiring mmap lock:
khugepaged_collapse_pte_mapped_thps() and hugepage_vma_revalidate() do so, for example. But retract_page_tables() did not: fix that.

The fix is for retract_page_tables() to check khugepaged_test_exit(), after acquiring mmap lock, before doing anything to the page table. Getting the mmap lock serializes with __mmput(), which briefly takes and drops it in __khugepaged_exit(); then the khugepaged_test_exit() check on mm_users makes sure we don't touch the page table once exit_mmap() might reach it, since exit_mmap() will be proceeding without mmap lock, not expecting anyone to be racing with it.

index ecefdba4b0dd..483c4573695a 100644
@@ -1251,6 +1251,7 @@ static void collect_mm_slot(struct mm_slot *mm_slot)
 static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 {
 	struct vm_area_struct *vma;
+	struct mm_struct *mm;
 	unsigned long addr;
 	pmd_t *pmd, _pmd;
 

@@ -1264,7 +1265,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			continue;
 		if (vma->vm_end < addr + HPAGE_PMD_SIZE)
 			continue;
-		pmd = mm_find_pmd(vma->vm_mm, addr);
+		mm = vma->vm_mm;
+		pmd = mm_find_pmd(mm, addr);
 		if (!pmd)
 			continue;
 		/*

@@ -1273,14 +1275,16 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 		 * re-fault. Not ideal, but it's more important to not disturb
 		 * the system too much.
 		 */
-		if (down_write_trylock(&vma->vm_mm->mmap_sem)) {
-			spinlock_t *ptl = pmd_lock(vma->vm_mm, pmd);
-			/* assume page table is clear */
-			_pmd = pmdp_collapse_flush(vma, addr, pmd);
-			spin_unlock(ptl);
-			up_write(&vma->vm_mm->mmap_sem);
-			mm_dec_nr_ptes(vma->vm_mm);
-			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
+		if (down_write_trylock(&mm->mmap_sem)) {
+			if (!khugepaged_test_exit(mm)) {
+				spinlock_t *ptl = pmd_lock(mm, pmd);
+				/* assume page table is clear */
+				_pmd = pmdp_collapse_flush(vma, addr, pmd);
+				spin_unlock(ptl);
+				mm_dec_nr_ptes(mm);
+				pte_free(mm, pmd_pgtable(_pmd));
+			}
+			up_write(&mm->mmap_sem);
 		}
 	}
 	i_mmap_unlock_write(mapping);