[PATCH v2 2/7] mm/khugepaged: stop swapping in page when VM_FAULT_RETRY occurs

[Miaohe Lin <linmiaohe@xxxxxxxxxx>]

When do_swap_page returns VM_FAULT_RETRY, we do not retry here and thus
swap entry will remain in pagetable. This will result in later failure.
So stop swapping in pages in this case to save cpu cycles. As A further
optimization, mmap_lock is released when __collapse_huge_page_swapin()
fails to avoid relocking mmap_lock. And "swapped_in++" is moved after
error handling to make it more accurate.

Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 mm/khugepaged.c | 32 ++++++++++++++------------------
 1 file changed, 14 insertions(+), 18 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8a103e0f8d2b..c6fc4eb8d77b 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -940,8 +940,8 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
  * Bring missing pages in from swap, to complete THP collapse.
  * Only done if khugepaged_scan_pmd believes it is worthwhile.
  *
- * Called and returns without pte mapped or spinlocks held,
- * but with mmap_lock held to protect against vma changes.
+ * Called and returns without pte mapped or spinlocks held.
+ * Note that if false is returned, mmap_lock will be released.
  */
 
 static bool __collapse_huge_page_swapin(struct mm_struct *mm,
@@ -968,27 +968,24 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 			pte_unmap(vmf.pte);
 			continue;
 		}
-		swapped_in++;
 		ret = do_swap_page(&vmf);
 
-		/* do_swap_page returns VM_FAULT_RETRY with released mmap_lock */
+		/*
+		 * do_swap_page returns VM_FAULT_RETRY with released mmap_lock.
+		 * Note we treat VM_FAULT_RETRY as VM_FAULT_ERROR here because
+		 * we do not retry here and swap entry will remain in pagetable
+		 * resulting in later failure.
+		 */
 		if (ret & VM_FAULT_RETRY) {
-			mmap_read_lock(mm);
-			if (hugepage_vma_revalidate(mm, haddr, &vma)) {
-				/* vma is no longer available, don't continue to swapin */
-				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
-				return false;
-			}
-			/* check if the pmd is still valid */
-			if (mm_find_pmd(mm, haddr) != pmd) {
-				trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
-				return false;
-			}
+			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
+			return false;
 		}
 		if (ret & VM_FAULT_ERROR) {
+			mmap_read_unlock(mm);
 			trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
 			return false;
 		}
+		swapped_in++;
 	}
 
 	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
@@ -1054,13 +1051,12 @@ static void collapse_huge_page(struct mm_struct *mm,
 	}
 
 	/*
-	 * __collapse_huge_page_swapin always returns with mmap_lock locked.
-	 * If it fails, we release mmap_lock and jump out_nolock.
+	 * __collapse_huge_page_swapin will return with mmap_lock released
+	 * when it fails. So we jump out_nolock directly in that case.
 	 * Continuing to collapse causes inconsistency.
 	 */
 	if (unmapped && !__collapse_huge_page_swapin(mm, vma, address,
 						     pmd, referenced)) {
-		mmap_read_unlock(mm);
 		goto out_nolock;
 	}
 
-- 
2.23.0