Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()

From: Omar Sandoval <osandov@osandov.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Dave Chinner <david@fromorbit.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH RESEND x3 v9 1/9] iov_iter: add copy_struct_from_iter()
Date: Fri, 25 Jun 2021 14:07:59 -0700	[thread overview]
Message-ID: <YNZFr7oJj1nkrwJY@relinquished.localdomain> (raw)
In-Reply-To: <CAHk-=wgH5pUbrL7CM5v6TWyNzDYpVM9k1qYCEgmY+b3Gx9nEAA@mail.gmail.com>

On Fri, Jun 25, 2021 at 09:16:15AM -0700, Linus Torvalds wrote:
> On Thu, Jun 24, 2021 at 8:38 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > Does it make any kind of sense to talk about doing this for buffered I/O,
> > given that we can't generate them for (eg) mmaped files?
> 
> Sure we can.
> 
> Or rather, some people might very well like to do it even for mutable
> data. In fact, _especially_ for mutable data.
> 
> You might want to do things like "write out the state I verified just
> a moment ago", and if it has changed since then, you *want* the result
> to be invalid because the checksums no longer match - in case somebody
> else changed the data you used for the state calculation and
> verification in the meantime. It's very much why you'd want a separate
> checksum in the first place.
> 
> Yeah, yeah,  you can - and people do - just do things like this with a
> separate checksum. But if you know that the filesystem has internal
> checksumming support _anyway_, you might want to use it, and basically
> say "use this checksum, if the data doesn't match when I read it back
> I want to get an IO error".
> 
> (The "data doesn't match" _could_ be just due to DRAM corruption etc,
> of course. Some people care about things like that. You want
> "verified" filesystem contents - it might not be about security, it
> might simply be about "I have validated this data and if it's not the
> same data any more it's useless and I need to re-generate it").
> 
> Am I a big believer in this model? No. Portability concerns (across
> OS'es, across filesystems, even just across backups on the same exact
> system) means that even if we did this, very few people would use it.
> 
> People who want this end up using an external checksum instead and do
> it outside of and separately from the actual IO, because then they can
> do it on existing systems.
> 
> So my argument is not "we want this". My argument is purely that some
> buffered filesystem IO case isn't actually any different from the
> traditional "I want access to the low-level sector hardware checksum
> data". The use cases are basically exactly the same.
> 
> Of course, basically nobody does that hw sector checksum either, for
> all the same reasons, even if it's been around for decades.
> 
> So my "checksum metadata interface" is not something I'm a big
> believer in, but I really don't think it's really all _that_ different
> from the whole "compressed format interface" that this whole patch
> series is about. They are pretty much the same thing in many ways.

I see the similarity in the sense that we basically want to pass some
extra metadata down with the read or write. So then do we want to add
preadv3/pwritev3 for encoded I/O now so that checksums can use it in the
future? The encoding metadata could go in this "struct io_how", either
directly or in a separate structure with a pointer in "struct io_how".
It could get messy with compat syscalls.