Re: bug in bio_map_user_iov

[Neil Brown <neilb@suse.de>]

On Wed, 17 Nov 2010 13:24:47 -0800
Michael Demmer <Michael.Demmer@riverbed.com> wrote:

> Hello all,
> 
> I've been doing some work with a Linux kernel module that enables zero-copy I/O to block devices using a custom user/kernel interface. In porting this from an older Linux kernel to a more modern release, I ran into an issue when interacting with MD devices that I traced back to what I believe to be a problem in bio_map_user_iov.
> 
> The problem and fix are described in the first attached patch. The second is a simple test module and user program which triggers the bug and validates the fix.
> 
> Thanks,
> -m
> 
> ps. This is my first attempt at pushing a patch upstream so please forgive any newbie mistakes.
> 

I think the real issue here is that bio_map_user is an interface that was
only intended to be used by bottom level devices like SCSI drivers etc.
It is a function that a device driver can use if it knows that it makes sense
to use it.

You are trying to use it as a generic interface that works for all block
devices, and it wasn't intended for that.

So while it is reasonably simple to 'fix' bio_map_user_iov, it is not
possible to 'fix' bio_map_kern_iov in the same way, because it doesn't have
access to the bdev at all.


So the question we should be asking is: are you really using the right
interface for the job?  Is bio_map_user something that you really should be
using?
And to answer that, we would need to know what you are trying to do.

And why isn't O_DIRECT a suitable zero-copy interface for I/O to block
devices?

NeilBrown