linux-ext4 - Re: [PATCH 2/3] ext4: Fix occasional generic/418 failure

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Date:   Tue, 13 Apr 2021 07:50:24 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jan Kara <jack@...e.cz>
Cc:     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 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?

Cheers,

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