Re: [PATCH v5 04/11] blksnap: header file of the module interface

[Christoph Hellwig]

On Wed, Jun 14, 2023 at 11:26:20AM +0200, Sergei Shtepa wrote:
> This code worked quite successfully for the veeamsnap module, on the
> basis of which blksnap was created. Indeed, such an allocation of an
> area on a block device using a file does not look safe.
> 
> We've already discussed this with Donald Buczek <buczek@xxxxxxxxxxxxx>.
> Link: https://github.com/veeam/blksnap/issues/57#issuecomment-1576569075
> And I have planned work on moving to a more secure ioctl in the future.
> Link: https://github.com/veeam/blksnap/issues/61
> 
> Now, thanks to Dave, it becomes clear to me how to solve this problem best.
> swapfile is a good example of how to do it right.

I don't actually think swapfile is a very good idea, in fact the Linux
swap code in general is not a very good place to look for inspirations
:)

IFF the usage is always to have a whole file for the diff storage the
over all API is very simple - just pass a fd to the kernel for the area,
and then use in-kernel direct I/O on it.  Now if that file should also
be able to reside on the same file system that the snapshot is taken
of things get a little more complicated, because writes to it also need
to automatically set the BIO_REFFED flag.  I have some ideas for that
and will share some draft code with you.