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

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Date: Thu, 2 May 2024 11:43:31 +1000
From: Dave Chinner <david@...morbit.com>
To: John Garry <john.g.garry@...cle.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk,
	brauner@...nel.org, jack@...e.cz, chandan.babu@...cle.com,
	willy@...radead.org, axboe@...nel.dk, martin.petersen@...cle.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
	ritesh.list@...il.com, mcgrof@...nel.org, p.raghav@...sung.com,
	linux-xfs@...r.kernel.org, catherine.hoang@...cle.com
Subject: Re: [PATCH v3 17/21] iomap: Atomic write support

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@...cle.com>
> > > ---
..
> > > +
> > >   		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?

-Dave.
-- 
Dave Chinner
david@...morbit.com