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Date:   Wed, 14 Jun 2023 18:43:10 +0200
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Subject: Re: [PATCH v5 04/11] blksnap: header file of the module interface
To:     Christoph Hellwig <hch@infradead.org>
CC:     Dave Chinner <david@fromorbit.com>, <axboe@kernel.dk>,
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        <linux-fsdevel@vger.kernel.org>,
        "Donald Buczek" <buczek@molgen.mpg.de>
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On 6/14/23 16:07, Christoph Hellwig wrote:
> 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
> ????

Perhaps. I haven't looked at the code yet. But I like the idea of
protecting the file from any access from the user-space, as it is
implemented for swapfile.

> 
> 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.

There is definitely no task to create a difference storage file on the
same block device for which the snapshot is being created. The file can
be created on any block device.

Still, the variant when a whole partition is allocated for the difference
storage can also be useful.

> I have some ideas for that and will share some draft code with you.

I'll be glad.