Re: [PATCH v3 17/21] iomap: Atomic write support

[John Garry]

On 02/05/2024 02:43, Dave Chinner wrote:
> On Wed, May 01, 2024 at 12:08:34PM +0100, John Garry wrote:
> > On 01/05/2024 02:47, Dave Chinner wrote:
> > > On Mon, Apr 29, 2024 at 05:47:42PM +0000, John Garry wrote:
> > > > Support atomic writes by producing a single BIO with REQ_ATOMIC flag set.
> > > >
> > > > We rely on the FS to guarantee extent alignment, such that an atomic write
> > > > should never straddle two or more extents. The FS should also check for
> > > > validity of an atomic write length/alignment.
> > > >
> > > > Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> > > > ---
> ...
> > > > +
> > > >    		bio->bi_private = dio;
> > > >    		bio->bi_end_io = iomap_dio_bio_end_io;
> > > > @@ -403,6 +407,12 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
> > > >    		}
> > > >    		n = bio->bi_iter.bi_size;
> > > > +		if (is_atomic && n != orig_count) {
> > > > +			/* This bio should have covered the complete length */
> > > > +			ret = -EINVAL;
> > > > +			bio_put(bio);
> > > > +			goto out;
> > > > +		}
> > >
> > > What happens now if we've done zeroing IO before this? I suspect we
> > > might expose stale data if the partial block zeroing converts the
> > > unwritten extent in full...
> >
> > We use iomap_dio.ref to ensure that __iomap_dio_rw() does not return until
> > any zeroing and actual sub-io block write completes. See iomap_dio_zero() ->
> > iomap_dio_submit_bio() -> atomic_inc(&dio->ref) callchain. I meant to add
> > such info to the commit message, as you questioned this previously.
>
> Yes, I get that. But my point is that we may have only done -part-
> of a block unaligned IO.
>
> This is effectively a failure from a bio_iov_iter_get_pages() call.
> What does the bio_iov_iter_get_pages() failure case do that this new
> failure case not do? Why does this case have different failure
> handling?

So you are saying that if we fail here (that is the (is_atomic && n != 
orig_count) check), anything unwritten in the atomic write region and 
zerotail region could expose stale data, right?

If so, I would say that we need to zero the complete unwritten atomic 
write and zerotail regions - similar to the bio_iov_iter_get_pages() 
failure handling - and still report an -EINVAL error.

How does that sound?

Thanks,
John