[PATCH v2 1/2] msync: support syncing a small part of the file

[Paolo Bonzini]

msync does not need to flush changes to the entire file, even with MS_SYNC.
Instead, it can use vfs_fsync_range to only synchronize a part of the file.
This is part of the specification; expecting msync to synchronize all the
file would take a very creative interpretation of the manual page as well
as the specification.

In addition, not all metadata has to be synced; msync is closer to
fdatasync than it is to fsync.  So, pass 1 to vfs_fsync_range.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Chris Friesen <chris.friesen@xxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
        v1->v2: fixed off-by-one in vall to vfs_fsync_range.

 mm/msync.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/msync.c b/mm/msync.c
index 632df45..505fe99 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -15,7 +15,7 @@
 #include <linux/sched.h>
 
 /*
- * MS_SYNC syncs the entire file - including mappings.
+ * MS_SYNC syncs the specified range - including mappings.
  *
  * MS_ASYNC does not start I/O (it used to, up to 2.5.67).
  * Nor does it marks the relevant pages dirty (it used to up to 2.6.17).
@@ -58,6 +58,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 	vma = find_vma(mm, start);
 	for (;;) {
 		struct file *file;
+		unsigned long next;
+		loff_t file_offset;
 
 		/* Still start < end. */
 		error = -ENOMEM;
@@ -77,18 +79,23 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 			goto out_unlock;
 		}
 		file = vma->vm_file;
-		start = vma->vm_end;
+		next = min(end, vma->vm_end);
 		if ((flags & MS_SYNC) && file &&
 				(vma->vm_flags & VM_SHARED)) {
+			file_offset = vma->vm_pgoff * PAGE_SIZE;
 			get_file(file);
 			up_read(&mm->mmap_sem);
-			error = vfs_fsync(file, 0);
+			error = vfs_fsync_range(file,
+				    start - vma->vm_start + file_offset,
+				    next - vma->vm_start + file_offset - 1, 1);
 			fput(file);
+			start = next;
 			if (error || start >= end)
 				goto out;
 			down_read(&mm->mmap_sem);
 			vma = find_vma(mm, start);
 		} else {
+			start = next;
 			if (start >= end) {
 				error = 0;
 				goto out_unlock;
-- 
1.7.1



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