linux-ext4 - Re: [PATCH v5 1/2] ext4: Discard preallocations before releasing group lock

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Message-ID: <20200924151238.GA3960@quack2.suse.cz>
Date:   Thu, 24 Sep 2020 17:12:38 +0200
From:   Jan Kara <jack@...e.cz>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Jan Kara <jack@...e.cz>, Ritesh Harjani <riteshh@...ux.ibm.com>,
        Ye Bin <yebin10@...wei.com>, adilger.kernel@...ger.ca,
        jack@...e.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v5 1/2] ext4: Discard preallocations before releasing
 group lock

On Thu 24-09-20 11:00:34, Theodore Y. Ts'o wrote:
> On Fri, Sep 18, 2020 at 11:56:53AM +0200, Jan Kara wrote:
> > On Fri 18-09-20 14:37:15, Ritesh Harjani wrote:
> > > 
> > > 
> > > On 9/16/20 5:08 PM, Ye Bin wrote:
> > > > From: Jan Kara <jack@...e.cz>
> > > > 
> > > > ext4_mb_discard_group_preallocations() can be releasing group lock with
> > > > preallocations accumulated on its local list. Thus although
> > > > discard_pa_seq was incremented and concurrent allocating processes will
> > > > be retrying allocations, it can happen that premature ENOSPC error is
> > > > returned because blocks used for preallocations are not available for
> > > > reuse yet. Make sure we always free locally accumulated preallocations
> > > > before releasing group lock.
> > > > 
> > > > Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
> > > > Signed-off-by: Jan Kara <jack@...e.cz>
> > > > Signed-off-by: Ye Bin <yebin10@...wei.com>
> > > > Reviewed-by: Ritesh Harjani <riteshh@...ux.ibm.com>
> > ...
> > > > +	/* if we still need more blocks and some PAs were used, try again */
> > > > +	if (free < needed && busy) {
> > > > +		ext4_unlock_group(sb, group);
> > > > +		cond_resched();
> > > > +		busy = 0;
> > > > +		/* Make sure we increment discard_pa_seq again */
> > > > +		needed -= free;
> > > > +		free = 0;
> > > 
> > > Oops sorry about getting back to this.
> > > But if we are making free 0 here so we may return a wrong free value
> > > when we return from this function. We should fix that by also accounting
> > > previous freed blocks at the time of final return from this function.
> > 
> > Ah, good catch! I'll send v2 with this fixed up.
> 
> Did you send a v2 of this patch?  I can't find it in my inbox...

Yeah, somehow I forgot to send it. I've sent it now:

https://lore.kernel.org/linux-ext4/20200924150959.4335-1-jack@suse.cz

Note that Ye Bin's patch will need trivial context fixup after applying
this...

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