linux-kernel - Re: [PATCH 2/2] ext4: correct the judgment of BUG in ext4_mb_normalize

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Date:   Tue, 24 May 2022 01:38:44 +0530
From:   Ritesh Harjani <ritesh.list@...il.com>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jack@...e.cz,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com,
        yebin10@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH 2/2] ext4: correct the judgment of BUG in
 ext4_mb_normalize_request

On 22/05/21 09:42PM, Baokun Li wrote:
> When either of the "start + size <= ac->ac_o_ex.fe_logical" or
> "start > ac->ac_o_ex.fe_logical" conditions is met, it indicates
> that the fe_logical is not in the allocated range.

Sounds about right to me based on the logic in ext4_mb_use_inode_pa().
We try to allocate/preallocate such that ac->ac_o_ex.fe_logical should fall
within the preallocated range. So if our start or start + size doesn't include
fe_logical then it is a bug in the ext4_mb_normalize_request() logic.

But should we be so harsh to hit a bug_on() or make it warn_on()?

Also did you run any fs tests with this change. Since it looks like this
logic existed since mballoc was introduced.


> In this case, it should be bug_ON.
>
> Fixes: dfe076c106f6 ("ext4: get rid of code duplication")

No, there is no issue with this patch. It correctly just removes the duplicate
logic.

> Signed-off-by: Baokun Li <libaokun1@...wei.com>
> ---
>  fs/ext4/mballoc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 32410b79b664..d0fb57970648 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -4190,7 +4190,7 @@ ext4_mb_normalize_request(struct ext4_allocation_context *ac,
>  	}
>  	rcu_read_unlock();
>
> -	if (start + size <= ac->ac_o_ex.fe_logical &&
> +	if (start + size <= ac->ac_o_ex.fe_logical ||
>  			start > ac->ac_o_ex.fe_logical) {
>  		ext4_msg(ac->ac_sb, KERN_ERR,
>  			 "start %lu, size %lu, fe_logical %lu",
> --
> 2.31.1
>