Re: Question regarding CIFS cache=loose behavior.

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2014-03-21 3:10 GMT+04:00 Jeff Layton <jlayton@xxxxxxxxxx>:
> On Fri, 21 Mar 2014 05:56:04 +0900
> Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>
>> Jeff Layton wrote:
>> > That's expected behavior. The kernel believes that the file is frozen in
>> > length so it returns short read() calls until the size is updated.
>>
>> The "size is updated" means "stat() detects the growth of file size",
>> doesn't it? Then, the former is expected behavior.
>>
>> >
>> > cache=loose is very much not recommended for use when you have multiple
>> > hosts accessing files on the server (or access by processes on the
>> > server itself). It only gives you "loose" cache coherency. The whole
>> > point of it is to allow the client to cache data even when the protocol
>> > says that it shouldn't.
>>
>> But why is the latter ( "read() returns non-0 when stat() detects the growth
>> of file size but the data actually read is '\0'" ) is expected behavior?
>> It sounds like a bug that the client caches '\0' (data nobody has ever wrote)
>> instead of '.' (data somebody wrote when the file size grew).
>>
>
> Yeah, that sounds wrong. What should happen is that the cache is
> invalidated when the size changes. It's possible there is a race in
> that code however. The locking around it is pretty sloppy...

When fstat() get a new file size it sets
CIFS_I(inode)->invalid_mapping to true but do not revalidate the
cache. Then generic_file_aio_read() reads the wrong data. I think we
need to check if CIFS_I(inode)->invalid_mapping is true and revalidate
the cache before calling generic_file_aio_read() in
file->f_ops->aio_read(). Now cache revalidation happens in lookup/open
and mmap codepaths only for cache=loose.

Of course, cache=loose is not recommended for this sort of work flow
and cache=strict should be used to provide a data coherency between
several machines.

-- 
Best regards,
Pavel Shilovsky.
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