# Re: \[PATCH RESEND x3 v9 1/9\] iov\_iter: add copy\_struct\_from\_iter()

- From: Josef Bacik \<josef@toxicpanda.com\>
- Date: 2021-07-19 16:11:39 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/07/210446
- Thread: https://ratatoskr.run/linux-fsdevel/2021/06/208723/t.md

```text
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 [ref] 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
```