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

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Message-ID: <ZIldkb1pwhNsSlfl@infradead.org>
Date:   Tue, 13 Jun 2023 23:26:25 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Sergei Shtepa <sergei.shtepa@...am.com>, axboe@...nel.dk,
        hch@...radead.org, corbet@....net, snitzer@...nel.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, dchinner@...hat.com,
        willy@...radead.org, dlemoal@...nel.org, linux@...ssschuh.net,
        jack@...e.cz, ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Donald Buczek <buczek@...gen.mpg.de>
Subject: Re: [PATCH v5 04/11] blksnap: header file of the module interface

On Wed, Jun 14, 2023 at 08:25:15AM +1000, Dave Chinner wrote:
> > + * Return: 0 if succeeded, negative errno otherwise.
> > + */
> > +#define IOCTL_BLKSNAP_SNAPSHOT_APPEND_STORAGE					\
> > +	_IOW(BLKSNAP, blksnap_ioctl_snapshot_append_storage,			\
> > +	     struct blksnap_snapshot_append_storage)
> 
> That's an API I'm extremely uncomfortable with. We've learnt the
> lesson *many times* that userspace physical mappings of underlying
> file storage are unreliable.
> 
> i.e.  This is reliant on userspace telling the kernel the physical
> mapping of the filesystem file to block device LBA space and then
> providing a guarantee (somehow) that the mapping will always remain
> unchanged. i.e. It's reliant on passing FIEMAP data from the
> filesystem to userspace and then back into the kernel without it
> becoming stale and somehow providing a guarantee that nothing (not
> even the filesystem doing internal garbage collection) will change
> it.

Hmm, I never thought of this API as used on files that somewhere
had a logical to physical mapping applied to them.

Sergey, is that the indtended use case?  If so we really should
be going through the file system using direct I/O.