linux-kernel - Re: [PATCH 13/21] ext4: remove dead check in mb_buddy_mark

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Message-ID: <Y+qUdLKXgWFrWlck@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date:   Tue, 14 Feb 2023 01:20:12 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     Kemeng Shi <shikemeng@...weicloud.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca, jack@...e.cz,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/21] ext4: remove dead check in mb_buddy_mark_free

On Fri, Feb 10, 2023 at 03:48:17AM +0800, Kemeng Shi wrote:
> We always adjust first to even number and adjust last to odd number, so
> first == last will never happen. Remove this dead check.
> 
> Signed-off-by: Kemeng Shi <shikemeng@...weicloud.com>
> ---
>  fs/ext4/mballoc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index bdabe0d81d4a..0fdbf16ac180 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1718,7 +1718,8 @@ static void mb_buddy_mark_free(struct ext4_buddy *e4b, int first, int last)
>  			break;
>  		order++;
>  
> -		if (first == last || !(buddy2 = mb_find_buddy(e4b, order, &max))) {
> +		buddy2 = mb_find_buddy(e4b, order, &max);
> +		if (!buddy2) {
>  			mb_clear_bits(buddy, first, last - first + 1);
>  			e4b->bd_info->bb_counters[order - 1] += last - first + 1;
>  			break;
> -- 
> 2.30.0
> 
Okay, so I checked the code and seems like you are right. There is can't be any
scenario where (first == last) after the calls to mb_buddy_adjust_border().

However, I'm a bit hesitant to give my Reviewed by since buddy algo is still a
bit confusing to me and I might be missing some weird edge case.  Maybe someone
can help double check this.