linux-kernel - Re: [PATCH v3 07/13] generic/1228: Add atomic write multi-fsblock O

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Message-ID: <20250723145423.GN2672039@frogsfrogsfrogs>
Date: Wed, 23 Jul 2025 07:54:23 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc: Zorro Lang <zlang@...hat.com>, fstests@...r.kernel.org,
	Ritesh Harjani <ritesh.list@...il.com>, john.g.garry@...cle.com,
	tytso@....edu, linux-xfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v3 07/13] generic/1228: Add atomic write multi-fsblock
 O_[D]SYNC tests

On Wed, Jul 23, 2025 at 07:23:58PM +0530, Ojaswin Mujoo wrote:
> On Thu, Jul 17, 2025 at 09:35:10AM -0700, Darrick J. Wong wrote:
> 
> <snip>
> 
> > > +verify_atomic_write() {
> > > +	if [[ "$1" == "shutdown" ]]
> > > +	then
> > > +		local do_shutdown=1
> > > +	fi
> > > +
> > > +	test $bytes_written -eq $awu_max || _fail "atomic write len=$awu_max assertion failed"
> > > +
> > > +	if [[ $do_shutdown -eq "1" ]]
> > > +	then
> > > +		echo "Shutting down filesystem" >> $seqres.full
> > > +		_scratch_shutdown >> $seqres.full
> > > +		_scratch_cycle_mount >>$seqres.full 2>&1 || _fail "remount failed for Test-3"
> > > +	fi
> > > +
> > > +	check_data_integrity
> > > +}
> > > +
> > > +mixed_mapping_test() {
> > > +	prep_mixed_mapping
> > > +
> > > +	echo "+ + Performing O_DSYNC atomic write from 0 to $awu_max" >> $seqres.full
> > > +	bytes_written=$($XFS_IO_PROG -dc "pwrite -DA -V1 -b $awu_max 0 $awu_max" $testfile | \
> > > +		        grep wrote | awk -F'[/ ]' '{print $2}')
> > > +
> > > +	verify_atomic_write $1
> > 
> > The shutdown happens after the synchronous write completes?  If so, then
> > what part of recovery is this testing?
> > 
> > --D
> 
> Right, it is mostly inspired by [1] where sometimes isize update could
> be lost after dio completion. Although this might not exactly be
> affected by atomic writes, we added it here out of caution.
> 
> [1] https://lore.kernel.org/fstests/434beffaf18d39f898518ea9eb1cea4548e77c3a.1695383715.git.ritesh.list@gmail.com/

Ah, so we're racing with background log flush then.  Would it improve
the potential failure detection rate to call shutdown right after the
pwrite, e.g.

$XFS_IO_PROG -dxc "pwrite -DA..." -c 'shutdown' $testfile

It can take a few milliseconds to walk down the bash functions and
fork/exec another child process.

--D

> > > +}
> > > +
>