Re: [PATCH] Loop device - Tracking page writes made to a loop devicethrough mmap

[Kandan Venkataraman]

On Tue, 13 Mar 2007, Christoph Hellwig wrote:

> On Wed, Mar 14, 2007 at 07:21:41AM +1100, Kandan Venkataraman wrote:
> > All comments have been taken care of.
> >
> > Description:
> >
> > A  file_operations structure variable called loop_fops is initialised with
> > the default block device file operations (def_blk_fops).
> > The mmap operation is overriden with a new function called loop_file_mmap.
>
> NACK.  block device driver should never ever play around with file
> operations themselves.  If you want functionality like the one you
> have please don't overload the loop driver, but start a new (character)
> driver doing specificaly what you want.  And even then I'm not sure
> we'd want functionality like this in the mainline tree, but at least
> we can have an open discussion if it's done properly.

The reason I chose the loop device was the following:

The loop device has backing storage and makes use of the page cache. The 
nopage method for all block device files and most disk files is 
implemented by filemap_nopage. This function uses the page cache. Thus, 
this allows me to mmap a large address space.

If I wrote a character device, I would have to store the contents in 
kernel memory as the function filemap_nopage is not suitable for character 
devices. This would severly restrict the size of my mmap.

I am out my depth here, any pointers on how to proceed would be very 
helpful

Thanks
Kandan
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