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

From: Josef Bacik <josef@toxicpanda.com>
To: Omar Sandoval <osandov@osandov.com>,
	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: Mon, 19 Jul 2021 11:44:54 -0400	[thread overview]
Message-ID: <84e667ce-0a26-3a2f-0fe8-4a56bfa43006@toxicpanda.com> (raw)
In-Reply-To: <YOXrmbi81Fr14fUV@relinquished.localdomain>

On 7/7/21 1:59 PM, Omar Sandoval wrote:
> On Fri, Jun 25, 2021 at 02:07:59PM -0700, Omar Sandoval wrote:
>> 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.
> 
> Ping. What's the path forward here? At this point, it seems like an
> ioctl is the path of least resistance.
> 

At this point we've been deadlocked on this for too long.  Put it in a btrfs 
IOCTL, if somebody wants to extend it generically in the future then godspeed, 
we can tie into that interface after the fact.  Thanks,

Josef