linux-kernel - Re: [PATCH v2] ext4: correct best extent lstart adjustment logic

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Message-ID: <20240201172405.zqeql5y2lzsi3o3y@quack3>
Date: Thu, 1 Feb 2024 18:24:05 +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, ojaswin@...ux.ibm.com,
	linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
	yangerkun@...wei.com, chengzhihao1@...wei.com, yukuai3@...wei.com,
	stable@...nel.org
Subject: Re: [PATCH v2] ext4: correct best extent lstart adjustment logic

On Thu 01-02-24 22:18:45, Baokun Li wrote:
> When yangerkun review commit 93cdf49f6eca ("ext4: Fix best extent lstart
> adjustment logic in ext4_mb_new_inode_pa()"), it was found that the best
> extent did not completely cover the original request after adjusting the
> best extent lstart in ext4_mb_new_inode_pa() as follows:
> 
>   original request: 2/10(8)
>   normalized request: 0/64(64)
>   best extent: 0/9(9)
> 
> When we check if best ex can be kept at start of goal, ac_o_ex.fe_logical
> is 2 less than the adjusted best extent logical end 9, so we think the
> adjustment is done. But obviously 0/9(9) doesn't cover 2/10(8), so we
> should determine here if the original request logical end is less than or
> equal to the adjusted best extent logical end.
> 
> In addition, add a comment stating when adjusted best_ex will not cover
> the original request, and remove the duplicate assertion because adjusting
> lstart makes no change to b_ex.fe_len.
> 
> Link: https://lore.kernel.org/r/3630fa7f-b432-7afd-5f79-781bc3b2c5ea@huawei.com
> Fixes: 93cdf49f6eca ("ext4: Fix best extent lstart adjustment logic in ext4_mb_new_inode_pa()")
> Cc: stable@...nel.org
> Signed-off-by: yangerkun <yangerkun@...wei.com>
> Signed-off-by: Baokun Li <libaokun1@...wei.com>

Looks good. Feel free to add:

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

								Honza

> ---
> V1->V2:
> 	Remove the problematic BUG_ON and add some comments.
> 
>  fs/ext4/mballoc.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 1ea6491b6b00..20cad0676aab 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -5148,10 +5148,16 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  			.fe_len = ac->ac_orig_goal_len,
>  		};
>  		loff_t orig_goal_end = extent_logical_end(sbi, &ex);
> +		loff_t o_ex_end = extent_logical_end(sbi, &ac->ac_o_ex);
>  
> -		/* we can't allocate as much as normalizer wants.
> -		 * so, found space must get proper lstart
> -		 * to cover original request */
> +		/*
> +		 * We can't allocate as much as normalizer wants, so we try
> +		 * to get proper lstart to cover the original request, except
> +		 * when the goal doesn't cover the original request as below:
> +		 *
> +		 * orig_ex:2045/2055(10), isize:8417280 -> normalized:0/2048
> +		 * best_ex:0/200(200) -> adjusted: 1848/2048(200)
> +		 */
>  		BUG_ON(ac->ac_g_ex.fe_logical > ac->ac_o_ex.fe_logical);
>  		BUG_ON(ac->ac_g_ex.fe_len < ac->ac_o_ex.fe_len);
>  
> @@ -5163,7 +5169,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  		 * 1. Check if best ex can be kept at end of goal (before
>  		 *    cr_best_avail trimmed it) and still cover original start
>  		 * 2. Else, check if best ex can be kept at start of goal and
> -		 *    still cover original start
> +		 *    still cover original end
>  		 * 3. Else, keep the best ex at start of original request.
>  		 */
>  		ex.fe_len = ac->ac_b_ex.fe_len;
> @@ -5173,7 +5179,7 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  			goto adjust_bex;
>  
>  		ex.fe_logical = ac->ac_g_ex.fe_logical;
> -		if (ac->ac_o_ex.fe_logical < extent_logical_end(sbi, &ex))
> +		if (o_ex_end <= extent_logical_end(sbi, &ex))
>  			goto adjust_bex;
>  
>  		ex.fe_logical = ac->ac_o_ex.fe_logical;
> @@ -5181,7 +5187,6 @@ ext4_mb_new_inode_pa(struct ext4_allocation_context *ac)
>  		ac->ac_b_ex.fe_logical = ex.fe_logical;
>  
>  		BUG_ON(ac->ac_o_ex.fe_logical < ac->ac_b_ex.fe_logical);
> -		BUG_ON(ac->ac_o_ex.fe_len > ac->ac_b_ex.fe_len);
>  		BUG_ON(extent_logical_end(sbi, &ex) > orig_goal_end);
>  	}
>  
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR