Re: [PATCH v7 16/19] iomap: Add done_before argument to iomap_dio_rw

[Darrick J. Wong]

On Fri, Aug 27, 2021 at 03:35:06PM -0700, Linus Torvalds wrote:
> On Fri, Aug 27, 2021 at 2:32 PM Darrick J. Wong <djwong@xxxxxxxxxx> wrote:
> >
> > No, because you totally ignored the second question:
> >
> > If the directio operation succeeds even partially and the PARTIAL flag
> > is set, won't that push the iov iter ahead by however many bytes
> > completed?
> >
> > We already finished the IO for the first page, so the second attempt
> > should pick up where it left off, i.e. the second page.
> 
> Darrick, I think you're missing the point.
> 
> It's the *return*value* that is the issue, not the iovec.
> 
> The iovec is updated as you say. But the return value from the async
> part is - without Andreas' patch - only the async part of it.
> 
> With Andreas' patch, the async part will now return the full return
> value, including the part that was done synchronously.
> 
> And the return value is returned from that async part, which somehow
> thus needs to know what predated it.

Aha, that was the missing piece, thank you.  I'd forgotten that
iomap_dio_complete_work calls iocb->ki_complete with the return value of
iomap_dio_complete, which means that the iomap_dio has to know if there
was a previous transfer that stopped short so that the caller could do
more work and resubmit.

> Could that pre-existing part perhaps be saved somewhere else? Very
> possibly. That 'struct iomap_dio' addition is kind of ugly. So maybe
> what Andreas did could be done differently.

There's probably a more elegant way for the ->ki_complete functions to
figure out how much got transferred, but that's sufficiently ugly and
invasive so as not to be suitable for a bug fix.

> But I think you guys are arguing past each other.

Yes, definitely.

--D

> 
>            Linus