[RFC] Dataloss on NFS-clients who modify an mmap()'d area after closing the file-descriptor

[Niels de Vos <ndevos-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

When an application on an NFS-client (tested with NFSv3) executes the
following steps, data written after the close() is never flushed to the
server:

1. open()
2. mmap()
3. close()
4. <modify data in the mmap'ed area>
5. munmap()

Dropping the caches (via /proc/sys/vm/drop_caches) or unmounting does not
result in the data being sent to the server.

The man-page for mmap (man 2 mmap) does mention that closing the file-
descriptor does not munmap() the area. Using the mmap'ed area after a
close() sound valid to me (even if it may be bad practice).

Investigation and checking showed that the NFS-client does not handle
munmap(), and only flushes on close(). To solve this problem, least two
solutions can be proposed:

a. f_ops->release() is called on munmap() as well as on close(),
    therefore release() can be used to flush data as well.
b. In the 'struct vm_operations_struct' add a .close to the
    'struct vm_area_struct' on calling mmap()/nfs_file_mmap() and flush
    the data in the new close() function.

Solution a. contains currently very few code changes:

--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -713,6 +713,8 @@ int nfs_open(struct inode *inode, struct file *filp)

  int nfs_release(struct inode *inode, struct file *filp)
  {
+       if (S_ISREG(inode->i_mode) && inode->i_mapping->nrpages != 0) {
+               nfs_sync_mapping(inode->i_mapping);
         nfs_file_clear_open_context(filp);
         return 0;
  }

The disadvantage is, that nfs_release() is called on close() too. That
means this causes a flushing of dirty pages, and just after that the
nfs_file_clear_open_context() might flush again. The advantage is that
it is possible (though not done at the moment) to return an error in
case flushing failed.

Solution b. does not provide an option to return an error, but does not
get called on each close():

--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -547,9 +547,17 @@ out:
  	return ret;
  }

+static void nfs_vm_close(struct vm_area_struct * vma)
+{
+	struct file *filp = vma->vm_file;
+
+	nfs_file_flush(filp, (fl_owner_t)filp);
+}
+
  static const struct vm_operations_struct nfs_file_vm_ops = {
  	.fault = filemap_fault,
  	.page_mkwrite = nfs_vm_page_mkwrite,
+	.close = nfs_vm_close,
  };

  static int nfs_need_sync_write(struct file *filp, struct inode *inode)

I would like some feedback on what solution is most acceptable, or any
other suggestions.

Many thanks,
Niels
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