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

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Message-Id: <0E3B5981-84C9-474C-91C2-311CB8B85C24@dilger.ca>
Date:	Mon, 31 Aug 2015 16:34:36 -0600
From:	Andreas Dilger <adilger@...ger.ca>
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 Aug 31, 2015, at 3:46 AM, Zhang Zhen <zhenzhang.zhang@...wei.com> 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.
> 
> 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. */

(typo) s/ramdon/random/

> +	if (free < needed && busy && tried < 20) {
> 		busy = 0;
> +		tried++;
> 		ext4_unlock_group(sb, group);
> 		cond_resched();
> 		goto repeat;

Looks OK otherwise.

Cheers, Andreas





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