HELP! vfs_readv() issue

HELP! vfs_readv() issue

[Xin Zhao]

I am writing a file system, but vfs_read() sometimes return 0. What
could cause this problem?

Please help!

Xin

Re: HELP! vfs_readv() issue

[Chris Wedgwood]

On Mon, May 15, 2006 at 07:57:21PM -0400, Xin Zhao wrote:

> I am writing a file system, but vfs_read() sometimes return 0. What
> could cause this problem?

EOF?

Re: HELP! vfs_readv() issue

[Xin Zhao]

Thank you for your care. What I am trying to do is to rewrite NFS in
the virtual machine environment so that network communication can be
replaced with inter-VM communication.

But after I remove the original rpc stuff, I ran into some strange
problem, including this one.  Interesting thing is that I noticed that
even with standard NFS implementation, it is still possible that
nfsd_read() return resp->count to be 0. At this time, eof is also
equal to 1. This seems to be right since NFSD already reach the end of
the file. But question is since 0 byte is read this time, NFS should
detect EOF in previous read. Why need one more read?

Xin

On 5/16/06, Chris Wedgwood <cw@f00f.org> wrote:
> On Mon, May 15, 2006 at 07:57:21PM -0400, Xin Zhao wrote:
>
> > I am writing a file system, but vfs_read() sometimes return 0. What
> > could cause this problem?
>
> EOF?
>

Re: HELP! vfs_readv() issue

[Avishay Traeger]

On Wed, 2006-05-17 at 17:44 -0400, Xin Zhao wrote:
> Thank you for your care. What I am trying to do is to rewrite NFS in
> the virtual machine environment so that network communication can be
> replaced with inter-VM communication.
> 
> But after I remove the original rpc stuff, I ran into some strange
> problem, including this one.  Interesting thing is that I noticed that
> even with standard NFS implementation, it is still possible that
> nfsd_read() return resp->count to be 0. At this time, eof is also
> equal to 1. This seems to be right since NFSD already reach the end of
> the file. But question is since 0 byte is read this time, NFS should
> detect EOF in previous read. Why need one more read?
> 
> Xin

How are you reading the file?  Some programs (I believe 'cat' is one of
them) will read a file until 0 is returned.  Try writing a small C
program to read a file until EOF and see if the behavior changes.

Avishay Traeger
http://www.fsl.cs.sunysb.edu/~avishay/

Re: HELP! vfs_readv() issue

[Petr Vandrovec]

Avishay Traeger wrote:
> On Wed, 2006-05-17 at 17:44 -0400, Xin Zhao wrote:
> 
>>Thank you for your care. What I am trying to do is to rewrite NFS in
>>the virtual machine environment so that network communication can be
>>replaced with inter-VM communication.
>>
>>But after I remove the original rpc stuff, I ran into some strange
>>problem, including this one.  Interesting thing is that I noticed that
>>even with standard NFS implementation, it is still possible that
>>nfsd_read() return resp->count to be 0. At this time, eof is also
>>equal to 1. This seems to be right since NFSD already reach the end of
>>the file. But question is since 0 byte is read this time, NFS should
>>detect EOF in previous read. Why need one more read?
>>
>>Xin
> 
> 
> How are you reading the file?  Some programs (I believe 'cat' is one of
> them) will read a file until 0 is returned.  Try writing a small C
> program to read a file until EOF and see if the behavior changes.

Returning 0 from read() is only situation when you can be sure you are at the 
end of file.  If you get short read(), it may be short due to EOF, but it may be 
short also because some error was hit - EIO and EFAULT are two which can occur 
almost always.  And only next read will either return that error (or some other 
error, or success if error condition disappeared meanwhile), or zero if it is 
really EOF.
								Petr