Re: [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>]

On 05/04/2012 08:29 PM, Myklebust, Trond wrote:
 > On Fri, 2012-05-04 at 18:03 +0200, Niels de Vos wrote:
 >> On 05/03/2012 07:26 PM, Myklebust, Trond wrote:
 >>   >  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...
 >>   >
 >>
 >> I've checked if the VM tags the pages as dirty:
 >> - f_ops->release() is called on munmap(). An added printk there, shows
 >>     that inode->i_state is set to I_DIRTY_PAGE.
 >> - mapping_tagged(filp->f_mapping, PAGECACHE_TAG_DIRTY) also returns true
 >>
 >>   From my understanding this is what the VM is expected to do, and the
 >> pages are marked dirty correctly.
 >>
 >> However, nfs_inode->ndirty and nfs_inode->ncommit are both 0. It is
 >> unclear to me how the VM is supposed to interact with the nfs_inode.
 >> Some clarification or suggestion what to look into would be much
 >> appreciated.
 >
 > The first time the page is touched, it will to trigger a ->pg_mkwrite(),
 > which in the case of NFS will set up the necessary tracking structures
 > to ensure that the page is written out using the correct credentials
 > etc. In the case of NFSv4, it will also ensure that the file doesn't get
 > closed on the server until the page is written out to disk.
 >
 > When the page is cleaned (i.e. something calls clear_page_dirty_for_io()
 > as part of a write to disk), the call to page_mkclean() is supposed to
 > re-write-protect the pte, ensuring that any future changes will
 > re-trigger pg_mkwrite().
 >
 > You should be able to check if/when nfs_vm_page_mkwrite() is triggered
 > using 'rpcdebug -m nfs -s pagecache' to turn on the NFS page cache
 > debugging printks.
 >

Many thanks for the explanation! At the moment I'm a little uncertain
where the problem lays, as the problem does not occur with more recent
kernels anymore. There likely was some invalid testing on my side :-/

I think I have to hunt down what changes were made and how this affects
the writing to mmap()'d files. The explanation you gave helps a lot in
understanding how NFS handles this all.

Thanks again, and sorry for any confusion,
Niels
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