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Date:   Fri,  1 May 2020 20:35:23 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     peterx@...hat.com, Alex Williamson <alex.williamson@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] mm/gup: Fix fixup_user_fault() on multiple retries

This part is overlooked when reworking the gup code on multiple retries.  When
we get the 2nd+ retry, we'll be with TRIED flag set.  Current code will bail
out on the 2nd retry because the !TRIED check will fail so the retry logic will
be skipped.  What's worse is that, it will also return zero which errornously
hints the caller that the page is faulted in while it's not.

The !TRIED flag check seems to not be needed even before the mutliple retries
change because if we get a VM_FAULT_RETRY, it must be the 1st retry, and we
should not have TRIED set for that.

Fix it by removing the !TRIED check, at the meantime check against fatal
signals properly before the page fault so we can still properly respond to the
user killing the process during retries.

CC: Alex Williamson <alex.williamson@...hat.com>
Fixes: 4426e945df58 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
Signed-off-by: Peter Xu <peterx@...hat.com>
---

I don't have a case to trigger the problem, so I only smoke tested the patch.
However I think this should be the right thing to do...  Please have a look,
thanks.
---
 mm/gup.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 50681f0286de..87a6a59fe667 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1218,6 +1218,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 	if (!vma_permits_fault(vma, fault_flags))
 		return -EFAULT;
 
+	if ((fault_flags & FAULT_FLAG_KILLABLE) &&
+	    fatal_signal_pending(current))
+		return -EINTR;
+
 	ret = handle_mm_fault(vma, address, fault_flags);
 	major |= ret & VM_FAULT_MAJOR;
 	if (ret & VM_FAULT_ERROR) {
@@ -1230,11 +1234,9 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 
 	if (ret & VM_FAULT_RETRY) {
 		down_read(&mm->mmap_sem);
-		if (!(fault_flags & FAULT_FLAG_TRIED)) {
-			*unlocked = true;
-			fault_flags |= FAULT_FLAG_TRIED;
-			goto retry;
-		}
+		*unlocked = true;
+		fault_flags |= FAULT_FLAG_TRIED;
+		goto retry;
 	}
 
 	if (tsk) {
-- 
2.26.2

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