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.
