linux-ext4 - Re: RWF_NOWAIT writes not failing when writing to a range with holes

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Message-ID: <20200708141706.GB5288@quack2.suse.cz>
Date:   Wed, 8 Jul 2020 16:17:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Filipe Manana <fdmanana@...il.com>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: RWF_NOWAIT writes not failing when writing to a range with holes

Hi!

On Mon 15-06-20 18:53:11, Filipe Manana wrote:
> I found out a bug in btrfs where a RWF_NOWRITE does not fail if we
> write to a range that starts with an extent followed by holes (since
> it requires allocating extent(s)).
> 
> When writing a test case for fstests I noticed xfs fails with -EAGAIN
> as expected, but ext4 succeeds just like btrfs currently does:
> 
> mkfs.ext4 -F /dev/sdb
> mount /dev/sdb /mnt
> 
> xfs_io -f -d -c "pwrite -S 0xab -b 256K 0 256K" /mnt/bar
> xfs_io -c "fpunch 64K 64K" /mnt/bar
> sync
> xfs_io -d -c "pwrite -N -V 1 -b 128K -S 0xfe 0 128K" /mnt/bar
> 
> Is this a known bug? Or is there a technical reason that makes it too
> expensive to check no extents will need to be allocated?

Thanks for report! This is actually a fallout of the conversion of ext4
direct IO code to iomap (commit 378f32bab37 "ext4: introduce direct I/O
write using iomap infrastructure"). I'll send a fix.

								Honza

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