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.
Any suggestions on correcting this (if it should be corrected) are
welcome.
Thanks again,
Niels
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