linux-kernel - [PATCH RFC v3 4/4] mm: gup: allow VM_FAULT

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Message-Id: <20181121062103.18835-5-peterx@redhat.com>
Date:   Wed, 21 Nov 2018 14:21:03 +0800
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Keith Busch <keith.busch@...el.com>,
        Jerome Glisse <jglisse@...hat.com>, peterx@...hat.com,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Huang Ying <ying.huang@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH RFC v3 4/4] mm: gup: allow VM_FAULT_RETRY for multiple times

This is the gup counterpart of the change that allows the VM_FAULT_RETRY
to happen for more than once.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/gup.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index a40111c742ba..7c3b3ab6be88 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -528,7 +528,10 @@ 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;
 	}
 
@@ -944,17 +947,23 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		/* VM_FAULT_RETRY triggered, so seek to the faulting offset */
 		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
+		 * with both FAULT_FLAG_ALLOW_RETRY and
 		 * FAULT_FLAG_TRIED.
 		 */
 		*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)
-- 
2.17.1