linux-ext4 - Re: [PATCH] fs/ext4: fix potential endless loop in ext4_mb_discard_group

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Date:	Wed, 16 Sep 2015 21:45:10 +0200
From:	Jan Kara <jack@...e.cz>
To:	Zhang Zhen <zhenzhang.zhang@...wei.com>
Cc:	"tytso@....edu" <tytso@....edu>, Jan Kara <jack@...e.cz>,
	adilger.kernel@...ger.ca,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] fs/ext4: fix potential endless loop in
 ext4_mb_discard_group_preallocations()

On Mon 31-08-15 17:46:24, Zhang Zhen wrote:
> In ext4_mb_discard_group_preallocations(), if free is always less than needed,
> and some PAs are always used in every loop, it will be endless loop.
> 
> Here we pick a random value to limit the max number of loop.

Were you able to trigger this in practice or is it just a theoretical
concern?

My slight concern is that in theory we could prematurely declare ENOSPC
with this patch since ext4_mb_discard_preallocations() doesn't reliably
discard all the preallocations anymore. But probably that's acceptable.
But we should add a comment before ext4_mb_discard_group_preallocations()
saying that the functions needn't free all the preallocations.

								Honza
> 
> Signed-off-by: Zhang Zhen <zhenzhang.zhang@...wei.com>
> ---
>  fs/ext4/mballoc.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 34b610e..553fbde 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3836,6 +3836,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
>  	int err;
>  	int busy = 0;
>  	int free = 0;
> +	int tried = 0;
> 
>  	mb_debug(1, "discard preallocation for group %u\n", group);
> 
> @@ -3886,9 +3887,11 @@ repeat:
>  		list_add(&pa->u.pa_tmp_list, &list);
>  	}
> 
> -	/* if we still need more blocks and some PAs were used, try again */
> -	if (free < needed && busy) {
> +	/* if we still need more blocks and some PAs were used, try again,
> +	   here 20 is a ramdon value. */
> +	if (free < needed && busy && tried < 20) {
>  		busy = 0;
> +		tried++;
>  		ext4_unlock_group(sb, group);
>  		cond_resched();
>  		goto repeat;
> -- 
> 1.9.1
> 
> 
> .
> 
> 
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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