linux-kernel - Re: [PATCH 1/1] quota: flush quota_release

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Message-ID: <ZzwlOukMjJtZxxgn@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Tue, 19 Nov 2024 11:12:18 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Jan Kara <jack@...e.cz>
Cc: linux-ext4@...r.kernel.org, Jan Kara <jack@...e.com>,
        Ritesh Harjani <ritesh.list@...il.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Disha Goel <disgoel@...ux.ibm.com>
Subject: Re: [PATCH 1/1] quota: flush quota_release_work upon quota writeback

On Mon, Nov 18, 2024 at 02:15:12PM +0100, Jan Kara wrote:
> On Sat 16-11-24 00:04:49, Ojaswin Mujoo wrote:
> > One of the paths quota writeback is called from is:
> > 
> > freeze_super()
> >   sync_filesystem()
> >     ext4_sync_fs()
> >       dquot_writeback_dquots()
> > 
> > Since we currently don't always flush the quota_release_work queue in
> > this path, we can end up with the following race:
> > 
> >  1. dquot are added to releasing_dquots list during regular operations.
> >  2. FS freeze starts, however, this does not flush the quota_release_work queue.
> >  3. Freeze completes.
> >  4. Kernel eventually tries to flush the workqueue while FS is frozen which
> >     hits a WARN_ON since transaction gets started during frozen state:
> > 
> >   ext4_journal_check_start+0x28/0x110 [ext4] (unreliable)
> >   __ext4_journal_start_sb+0x64/0x1c0 [ext4]
> >   ext4_release_dquot+0x90/0x1d0 [ext4]
> >   quota_release_workfn+0x43c/0x4d0
> > 
> > Which is the following line:
> > 
> >   WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);
> > 
> > Which ultimately results in generic/390 failing due to dmesg
> > noise. This was detected on powerpc machine 15 cores.
> > 
> > To avoid this, make sure to flush the workqueue during
> > dquot_writeback_dquots() so we dont have any pending workitems after
> > freeze.
> > 
> > Reported-by: Disha Goel <disgoel@...ux.ibm.com>
> > Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
> 
> Thanks for debugging this!
> 
> > diff --git a/fs/quota/dquot.c b/fs/quota/dquot.c
> > index 3dd8d6f27725..2782cfc8c302 100644
> > --- a/fs/quota/dquot.c
> > +++ b/fs/quota/dquot.c
> > @@ -729,6 +729,8 @@ int dquot_writeback_dquots(struct super_block *sb, int type)
> >  			sb->dq_op->write_info(sb, cnt);
> >  	dqstats_inc(DQST_SYNCS);
> >  
> > +	flush_delayed_work(&quota_release_work);
> > +
> 
> I'd rather do this at the start of dquot_writeback_dquots(). Chances are
> this saves some retry loops in the dirty list iterations. That being said I

Hi Jan, thanks for review :)


> don't think this is enough as I'm thinking about it. iput() can be called
> anytime while the filesystem is frozen (just freeze the filesystem and do
> echo 3 >/proc/sys/vm/drop_caches) which will consequently call dquot_drop()
> -> dqput(). This should not be really freeing the dquot on-disk structure
> (the inode itself is still accounted there) but nevertheless it may end up
> dropping the last dquot in-memory reference and ext4_release_dquot() will
> call ext4_journal_start() and complain. So I think on top of this patch
> which makes sense on its own and deals with 99.9% of cases, we also need
> ext4 specific fix which uses sb_start_intwrite() to get freeze protection
> in ext4_release_dquot() (and in principle we always needed this, delayed
> dquot releasing does not influence this particular problem). Some care will
> be needed if the transaction is already started when ext4_release_dquot()
> is called - you can take inspiration in how ext4_evict_inode() handles
> this.

That's a good point Jan, this could indeed happen if we drop caches
destroying an inode pinned in the lru cache. Thanks for the pointers,
I'll try to look into hardening ext4_release_dquot() as you suggested
and send a v2.

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