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Date:   Tue, 13 Apr 2021 11:11:22 +0200
From:   Jan Kara <jack@...e.cz>
To:     Dave Chinner <david@...morbit.com>
Cc:     Jan Kara <jack@...e.cz>, Ted Tso <tytso@....edu>,
        linux-ext4@...r.kernel.org, Eric Whitney <enwlinux@...il.com>,
        linux-fsdevel@...r.kernel.org,
        "Darrick J . Wong" <djwong@...nel.org>
Subject: Re: [PATCH 2/3] ext4: Fix occasional generic/418 failure

On Tue 13-04-21 07:50:24, Dave Chinner wrote:
> On Mon, Apr 12, 2021 at 12:23:32PM +0200, Jan Kara wrote:
> > Eric has noticed that after pagecache read rework, generic/418 is
> > occasionally failing for ext4 when blocksize < pagesize. In fact, the
> > pagecache rework just made hard to hit race in ext4 more likely. The
> > problem is that since ext4 conversion of direct IO writes to iomap
> > framework (commit 378f32bab371), we update inode size after direct IO
> > write only after invalidating page cache. Thus if buffered read sneaks
> > at unfortunate moment like:
> > 
> > CPU1 - write at offset 1k                       CPU2 - read from offset 0
> > iomap_dio_rw(..., IOMAP_DIO_FORCE_WAIT);
> >                                                 ext4_readpage();
> > ext4_handle_inode_extension()
> > 
> > the read will zero out tail of the page as it still sees smaller inode
> > size and thus page cache becomes inconsistent with on-disk contents with
> > all the consequences.
> > 
> > Fix the problem by moving inode size update into end_io handler which
> > gets called before the page cache is invalidated.
> 
> Confused.
> 
> This moves all the inode extension stuff into the completion
> handler, when all that really needs to be done is extending
> inode->i_size to tell the world there is data up to where the
> IO completed. Actually removing the inode from the orphan list
> does not need to be done in the IO completion callback, because...
> 
> >  	if (ilock_shared)
> >  		iomap_ops = &ext4_iomap_overwrite_ops;
> > -	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
> > -			   (unaligned_io || extend) ? IOMAP_DIO_FORCE_WAIT : 0);
> > -	if (ret == -ENOTBLK)
> > -		ret = 0;
> > -
> >  	if (extend)
> > -		ret = ext4_handle_inode_extension(inode, offset, ret, count);
> > +		dio_ops = &ext4_dio_extending_write_ops;
> >  
> > +	ret = iomap_dio_rw(iocb, from, iomap_ops, dio_ops,
> > +			   (extend || unaligned_io) ? IOMAP_DIO_FORCE_WAIT : 0);
>                             ^^^^^^                    ^^^^^^^^^^^^^^^^^^^ 
> 
> .... if we are doing an extending write, we force DIO to complete
> before returning. Hence even AIO will block here on an extending
> write, and hence we can -always- do the correct post-IO completion
> orphan list cleanup here because we know a) the original IO size and
> b) the amount of data that was actually written.
> 
> Hence all that remains is closing the buffered read vs invalidation
> race. All this requires is for the dio write completion to behave
> like XFS where it just does the inode->i_size update for extending
> writes. THis means the size is updated before the invalidation, and
> hence any read that occurs after the invalidation but before the
> post-eof blocks have been removed will see the correct size and read
> the tail page(s) correctly. This closes the race window, and the
> caller can still handle the post-eof block cleanup as it does now.
> 
> Hence I don't see any need for changing the iomap infrastructure to
> solve this problem. This seems like the obvious solution to me, so
> what am I missing?

All that you write above is correct. The missing piece is: If everything
succeeded and all the cleanup we need is removing inode from the orphan
list (common case), we want to piggyback that orphan list removal into the
same transaction handle as the update of the inode size. This is just a
performance thing, you are absolutely right we could also do the orphan
cleanup unconditionally in ext4_dio_write_iter() and thus avoid any changes
to the iomap framework.

OK, now that I write about this, maybe I was just too hung up on the
performance improvement. Probably a better way forward is that I just fix
the data corruption bug only inside ext4 (that will be also much easier to
backport) and then submit the performance improvement modifying iomap if I
can actually get performance data justifying it. Thanks for poking into
this :)

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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