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Date:   Wed, 15 Jun 2022 10:40:17 +0200
From:   Lukas Czerner <lczerner@redhat.com>
To:     Jinke Han <hanjinke.666@bytedance.com>
Cc:     tytso@mit.edu, adilger.kernel@dilger.ca,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ext4: fix trim range leak
Message-ID: <20220615084017.xwexup5ckrrpevhe@fedora>
References: <20220614044647.21846-1-hanjinke.666@bytedance.com>
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On Tue, Jun 14, 2022 at 12:46:47PM +0800, Jinke Han wrote:
> From: hanjinke <hanjinke.666@bytedance.com>
> 
> When release group lock, a large number of blocks may be alloc from
> the group(e.g. not from the rest of target trim range). This may
> lead end of the loop and leave the rest of trim range unprocessed.

Hi,

you're correct. Indeed it's possible to miss some of the blocks this
way.

But I wonder how much of a problem this actually is? I'd think that the
optimization you just took out is very usefull, especially with larger
minlen and more fragmented free space it'll save us a lot of cycles.
Do you have any performance numbers for this change?

Perhaps we don't have to remove it completely, rather zero the
free_count every time bb_free changes? Would that be worth it?

-Lukas

> 
> Signed-off-by: hanjinke <hanjinke.666@bytedance.com>
> ---
>  fs/ext4/mballoc.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 9f12f29bc346..45eb9ee20947 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -6345,14 +6345,13 @@ static int ext4_try_to_trim_range(struct super_block *sb,
>  __acquires(ext4_group_lock_ptr(sb, e4b->bd_group))
>  __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
>  {
> -	ext4_grpblk_t next, count, free_count;
> +	ext4_grpblk_t next, count;
>  	void *bitmap;
>  
>  	bitmap = e4b->bd_bitmap;
>  	start = (e4b->bd_info->bb_first_free > start) ?
>  		e4b->bd_info->bb_first_free : start;
>  	count = 0;
> -	free_count = 0;
>  
>  	while (start <= max) {
>  		start = mb_find_next_zero_bit(bitmap, max + 1, start);
> @@ -6367,7 +6366,6 @@ __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
>  				break;
>  			count += next - start;
>  		}
> -		free_count += next - start;
>  		start = next + 1;
>  
>  		if (fatal_signal_pending(current)) {
> @@ -6381,8 +6379,6 @@ __releases(ext4_group_lock_ptr(sb, e4b->bd_group))
>  			ext4_lock_group(sb, e4b->bd_group);
>  		}
>  
> -		if ((e4b->bd_info->bb_free - free_count) < minblocks)
> -			break;
>  	}
>  
>  	return count;
> -- 
> 2.20.1
>