linux-ext4 - Re: [PATCH 4/4] ext4: avoid dividing by 0 in mb_update_avg_fragment

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Date: Mon, 18 Dec 2023 15:43:42 +0100
From: Jan Kara <jack@...e.cz>
To: Baokun Li <libaokun1@...wei.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca,
	jack@...e.cz, ritesh.list@...il.com, linux-kernel@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com, yukuai3@...wei.com,
	stable@...r.kernel.org
Subject: Re: [PATCH 4/4] ext4: avoid dividing by 0 in
 mb_update_avg_fragment_size() when block bitmap corrupt

On Mon 18-12-23 22:18:14, Baokun Li wrote:
> When bb_free is not 0 but bb_fragments is 0, return directly to avoid
> system crash due to division by zero.

How could this possibly happen? bb_fragments is the number of free space
extents and bb_free is the number of free blocks. No free space extents =>
no free blocks seems pretty obvious? You can see the logic in
ext4_mb_generate_buddy()...

								Honza

> 
> Fixes: 83e80a6e3543 ("ext4: use buckets for cr 1 block scan instead of rbtree")
> CC: stable@...r.kernel.org
> Signed-off-by: Baokun Li <libaokun1@...wei.com>
> ---
>  fs/ext4/mballoc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 2fbee0f0f5c3..e2a167240335 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -845,6 +845,9 @@ mb_update_avg_fragment_size(struct super_block *sb, struct ext4_group_info *grp)
>  	if (!test_opt2(sb, MB_OPTIMIZE_SCAN) || grp->bb_free == 0)
>  		return;
>  
> +	if (unlikely(grp->bb_fragments == 0))
> +		return;
> +
>  	new_order = mb_avg_fragment_size_order(sb,
>  					grp->bb_free / grp->bb_fragments);
>  	if (new_order == grp->bb_avg_fragment_size_order)
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR