On Thu, 2012-05-03 at 19:07 +0200, Niels de Vos wrote:
> On 05/03/2012 05:43 PM, Myklebust, Trond wrote:
> > On Thu, 2012-05-03 at 17:34 +0200, Niels de Vos wrote:
> >> 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.
> >
> > Neither solution is acceptable. This isn't a close-to-open cache
> > consistency issue.
> >
> > The syntax of mmap() for both block and NFS mounts is the same: writes
> > are not guaranteed to hit the disk until your application explicitly
> > calls msync().
> >
>
> Okay, that makes sense. But if the application never calls msync(), and
> just munmap()'s the area, when should the changes be written? I did not
> expect that unmounting just disregards the data.
That suggests that the VM is failing to dirty the pages on munmap()
before releasing the vma->vm_file. If so, then that would be a VM bug...
--
Trond Myklebust
Linux NFS client maintainer
NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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