linux-ext4 - Re: [PATCH v5 2/2] ext4: Fix dead loop in ext4_mb_new

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Date:   Thu, 24 Sep 2020 10:59:33 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Ye Bin <yebin10@...wei.com>
Cc:     riteshh@...ux.ibm.com, jack@...e.cz, adilger.kernel@...ger.ca,
        jack@...e.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v5 2/2] ext4: Fix dead loop in ext4_mb_new_blocks

On Wed, Sep 16, 2020 at 07:38:59PM +0800, Ye Bin wrote:
> As we test disk offline/online with running fsstress, we find fsstress
> process is keeping running state.
> kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114
> ....
> kworker/u32:3-262   [004] ...1   140.787471: ext4_mb_discard_preallocations: dev 8,32 needed 114
> 
> ext4_mb_new_blocks
> repeat:
>         ext4_mb_discard_preallocations_should_retry(sb, ac, &seq)
>                 freed = ext4_mb_discard_preallocations
>                         ext4_mb_discard_group_preallocations
>                                 this_cpu_inc(discard_pa_seq);
>                 ---> freed == 0
>                 seq_retry = ext4_get_discard_pa_seq_sum
>                         for_each_possible_cpu(__cpu)
>                                 __seq += per_cpu(discard_pa_seq, __cpu);
>                 if (seq_retry != *seq) {
>                         *seq = seq_retry;
>                         ret = true;
>                 }
> 
> As we see seq_retry is sum of discard_pa_seq every cpu, if
> ext4_mb_discard_group_preallocations return zero discard_pa_seq in this
> cpu maybe increase one, so condition "seq_retry != *seq" have always
> been met.
> Ritesh Harjani suggest to in ext4_mb_discard_group_preallocations function we
> only increase discard_pa_seq when there is some PA to free.
> 
> Fixes: 07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to improve ENOSPC handling")
> Signed-off-by: Ye Bin <yebin10@...wei.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Reviewed-by: Ritesh Harjani <riteshh@...ux.ibm.com>

Thanks, applied.

						- Ted