[PATCH] Sync only the requested range in msync

[Matthew Wilcox <matthew.r.wilcox@intel.com>]

[untested.  posted because it keeps coming up at lsfmm/collab]

msync() currently syncs more than POSIX requires or BSD or Solaris
implement.  It is supposed to be equivalent to fdatasync(), not fsync(),
and it is only supposed to sync the portion of the file that overlaps
the range passed to msync.

If the VMA is non-linear, fall back to syncing the entire file, but we
still optimise to only fdatasync() the entire file, not the full fsync().

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
---
 mm/msync.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/msync.c b/mm/msync.c
index 632df45..a5c6736 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -58,6 +58,7 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 	vma = find_vma(mm, start);
 	for (;;) {
 		struct file *file;
+		loff_t fstart, fend;
 
 		/* Still start < end. */
 		error = -ENOMEM;
@@ -77,12 +78,17 @@ 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);
+		fend = fstart + (min(end, vma->vm_end) - start) - 1;
 		start = vma->vm_end;
 		if ((flags & MS_SYNC) && file &&
 				(vma->vm_flags & VM_SHARED)) {
 			get_file(file);
 			up_read(&mm->mmap_sem);
-			error = vfs_fsync(file, 0);
+			if (vma->vm_flags & VM_NONLINEAR)
+				error = vfs_fsync(file, 1);
+			else
+				error = vfs_fsync_range(file, fstart, fend, 1);
 			fput(file);
 			if (error || start >= end)
 				goto out;
-- 
1.9.0