[PATCH] msync: fix incorrect fstart calculation

Fix a regression caused by Commit 7fc34a62ca mm/msync.c: sync only
the requested range in msync().
xfstests generic/075 fail occured on ext4 data=journal mode because
the intended range was not syncing due to wrong fstart calculation.

Cc: Matthew Wilcox <redacted>
Cc: Lukáš Czerner <redacted>
Reported-by: Eric Whitney <redacted>
Signed-off-by: Namjae Jeon <redacted>
Signed-off-by: Ashish Sangwan <redacted>
---
 mm/msync.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/msync.c b/mm/msync.c
index a5c6736..ad97dce 100644
--- a/mm/msync.c
+++ b/mm/msync.c

@@ -78,7 +78,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 			goto out_unlock;
 		}
 		file = vma->vm_file;
-		fstart = start + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
+		fstart = (start - vma->vm_start) +
+			 ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
 		fend = fstart + (min(end, vma->vm_end) - start) - 1;
 		start = vma->vm_end;
 		if ((flags & MS_SYNC) && file &&

-- 
1.7.11-rc0

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