linux-ext4 - Re: [PATCH v4] ext4: Fix dead loop in ext4_mb_new

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Date:   Tue, 15 Sep 2020 14:11:22 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ye Bin <yebin10@...wei.com>
Cc:     riteshh@...ux.ibm.com, jack@...e.cz, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH v4]  ext4: Fix dead loop in ext4_mb_new_blocks

On Mon 14-09-20 18:47:42, 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>

The patch looks good to me. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

But as I mentioned in my previous reply I also think the attached patch
also needs to be merged to avoid premature ENOSPC errors (which your change
makes somewhat more likely). Ritesh do you agree?

								Honza


> ---
>  fs/ext4/mballoc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 132c118d12e1..ff47347012f4 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4189,7 +4189,6 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
>  	INIT_LIST_HEAD(&list);
>  repeat:
>  	ext4_lock_group(sb, group);
> -	this_cpu_inc(discard_pa_seq);
>  	list_for_each_entry_safe(pa, tmp,
>  				&grp->bb_prealloc_list, pa_group_list) {
>  		spin_lock(&pa->pa_lock);
> @@ -4206,6 +4205,9 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
>  		/* seems this one can be freed ... */
>  		ext4_mb_mark_pa_deleted(sb, pa);
>  
> +		if (!free)
> +			this_cpu_inc(discard_pa_seq);
> +
>  		/* we can trust pa_free ... */
>  		free += pa->pa_free;
>  
> -- 
> 2.25.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

View attachment "0001-ext4-Discard-preallocations-before-releasing-group-l.patch" of type "text/x-patch" (2140 bytes)