linux-ext4 - Re: generic/232 test failures on 4.14-rc1

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Date:   Tue, 26 Sep 2017 17:41:52 -0400
From:   Eric Whitney <enwlinux@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Eric Whitney <enwlinux@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: generic/232 test failures on 4.14-rc1

* Jan Kara <jack@...e.cz>:
> On Mon 25-09-17 15:59:46, Jan Kara wrote:
> > On Thu 21-09-17 11:48:46, Eric Whitney wrote:
> > > I'm seeing generic/232 fail from time to time when running a 4.14-rc1 kernel
> > > on xfstest-bld's most recent kvm-xfstests test appliance.  In one set of
> > > trials, it failed in the same manner 4 out of 10 times when running the 4k test
> > > configuration for ext4.
> > > 
> > > The failure bisects to "quota: Do not acquire dqio_sem for dquot overwrites in
> > > v2 format" (ab2b86360f6e).  When this patch was reverted in a 4.14-rc1 kernel,
> > > the failure did not reoccur in a series of 20 trials.
> > 
> > Thanks for debugging this! I'd just note that the commit hash of that
> > change is different for me - d2faa415166b2883428efa92f451774ef44373ac.
> > 
> > > Example output from the failed test:
> > > 
> > > QA output created by 232
> > > 
> > > Testing fsstress
> > > 
> > > seed = S
> > > Comparing user usage
> > > 218a219
> > > > #3740     --       4       0       0              1     0     0       
> > > 245a247
> > > > #45       --       0       0       0              1     0     0     
> > > 
> > > Note:  I'm also seeing a similar failure for generic/233, but the patch
> > > containing the root cause likely comes somewhere after ab2b86360f6e.  I'll post
> > > another bug report once I locate it.
> > 
> > I'll try to debug this further. Thanks for report!
> 
> Attached patch fixes the problem for me. I'll merge it through my tree.
> 
> 								Honza
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR

> From a0ae41c2a9c204374eafd24a928e4352841bd905 Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@...e.cz>
> Date: Tue, 26 Sep 2017 10:36:05 +0200
> Subject: [PATCH] quota: Fix quota corruption with generic/232 test
> 
> Eric has reported that since commit d2faa415166b "quota: Do not acquire
> dqio_sem for dquot overwrites in v2 format" test generic/232
> occasionally fails due to quota information being incorrect. Indeed that
> commit was too eager to remove dqio_sem completely from the path that
> just overwrites quota structure with updated information. Although that
> is innocent on its own, another process that inserts new quota structure
> to the same block can perform read-modify-write cycle of that block thus
> effectively discarding quota information update if they race in a wrong
> way.
> 
> Fix the problem by acquiring dqio_sem for reading for overwrites of
> quota structure. Note that it *is* possible to completely avoid taking
> dqio_sem in the overwrite path however that will require modifying path
> inserting / deleting quota structures to avoid RMW cycles of the full
> block and for now it is not clear whether it is worth the hassle.
> 
> Fixes: d2faa415166b2883428efa92f451774ef44373ac
> Reported-by: Eric Whitney <enwlinux@...il.com>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/quota/quota_v2.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/quota/quota_v2.c b/fs/quota/quota_v2.c
> index c0187cda2c1e..a73e5b34db41 100644
> --- a/fs/quota/quota_v2.c
> +++ b/fs/quota/quota_v2.c
> @@ -328,12 +328,16 @@ static int v2_write_dquot(struct dquot *dquot)
>  	if (!dquot->dq_off) {
>  		alloc = true;
>  		down_write(&dqopt->dqio_sem);
> +	} else {
> +		down_read(&dqopt->dqio_sem);
>  	}
>  	ret = qtree_write_dquot(
>  			sb_dqinfo(dquot->dq_sb, dquot->dq_id.type)->dqi_priv,
>  			dquot);
>  	if (alloc)
>  		up_write(&dqopt->dqio_sem);
> +	else
> +		up_read(&dqopt->dqio_sem);
>  	return ret;
>  }
>  
> -- 
> 2.12.3
> 

Hi Honza:

That patch works for me - 100 out of 100 trials of generic/232 passed
successfully running a modified 4.14-rc1 kernel on kvm-xfstests' ext4 4k
test configuration.

Tested-by: Eric Whitney <enwlinux@...il.com>

Thanks!
Eric