[PATCH v4 08/10] mm/gup: Allow VM_FAULT_RETRY for multiple times

[Peter Xu]

This is the gup counterpart of the change that allows the
VM_FAULT_RETRY to happen for more than once.  One thing to mention is
that we must check the fatal signal here before retry because the GUP
can be interrupted by that, otherwise we can loop forever.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/gup.c     | 27 +++++++++++++++++++++------
 mm/hugetlb.c |  6 ++++--
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e60d32f1674d..d2811bb15a25 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -644,7 +644,10 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
-		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
+		/*
+		 * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
+		 * can co-exist
+		 */
 		fault_flags |= FAULT_FLAG_TRIED;
 	}
 
@@ -994,7 +997,6 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 		down_read(&mm->mmap_sem);
 		if (!(fault_flags & FAULT_FLAG_TRIED)) {
 			*unlocked = true;
-			fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			fault_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
@@ -1069,17 +1071,30 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		if (likely(pages))
 			pages += ret;
 		start += ret << PAGE_SHIFT;
+		lock_dropped = true;
 
+retry:
 		/*
 		 * Repeat on the address that fired VM_FAULT_RETRY
-		 * without FAULT_FLAG_ALLOW_RETRY but with
-		 * FAULT_FLAG_TRIED.
+		 * with both FAULT_FLAG_ALLOW_RETRY and
+		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
+		 * by fatal signals, so we need to check it before we
+		 * start trying again otherwise it can loop forever.
 		 */
+
+		if (fatal_signal_pending(current))
+			break;
+
 		*locked = 1;
-		lock_dropped = true;
 		down_read(&mm->mmap_sem);
+
 		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
-				       pages, NULL, NULL);
+				       pages, NULL, locked);
+		if (!*locked) {
+			/* Continue to retry until we succeeded */
+			BUG_ON(ret != 0);
+			goto retry;
+		}
 		if (ret != 1) {
 			BUG_ON(ret > 1);
 			if (!pages_done)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 31c2a6275023..d0c98cff5b0f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4347,8 +4347,10 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_RETRY_NOWAIT;
 			if (flags & FOLL_TRIED) {
-				VM_WARN_ON_ONCE(fault_flags &
-						FAULT_FLAG_ALLOW_RETRY);
+				/*
+				 * Note: FAULT_FLAG_ALLOW_RETRY and
+				 * FAULT_FLAG_TRIED can co-exist
+				 */
 				fault_flags |= FAULT_FLAG_TRIED;
 			}
 			ret = hugetlb_fault(mm, vma, vaddr, fault_flags);
-- 
2.21.0