msync: fix incorrect fstart calculation

Fix a regression caused by 7fc34a62ca44 ("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.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Reported-by: Eric Whitney <enwlinux@gmail.com>
Tested-by: Eric Whitney <enwlinux@gmail.com>
Acked-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Tested-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/mm/msync.c b/mm/msync.c
index a5c673669ca6..992a1673d488 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 &&