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

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Date:   Fri, 18 Sep 2020 11:56:53 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     Ye Bin <yebin10@...wei.com>, adilger.kernel@...ger.ca,
        jack@...e.com, linux-ext4@...r.kernel.org, jack@...e.cz,
        tytso@....edu
Subject: Re: [PATCH v5 1/2] ext4: Discard preallocations before releasing
 group lock

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.

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