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.
Cheers,
Niels
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