linux-ext4 - Re: [PATCH] ext4: optimize the implementation of ext4_mb_good

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Date:   Thu, 13 Aug 2020 21:01:18 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     brookxu <brookxu.cn@...il.com>, adilger.kernel@...ger.ca,
        tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: optimize the implementation of ext4_mb_good_group()



On 8/7/20 7:31 PM, brookxu wrote:
> It might be better to adjust the code in two places:
> 1. Determine whether grp is currupt or not should be placed first.
> 2. (cr<=2 && free <ac->ac_g_ex.fe_len)should may belong to the crx
>     strategy, and it may be more appropriate to put it in the
>     subsequent switch statement block. For cr1, cr2, the conditions
>     in switch potentially realize the above judgment. For cr0, we
>     should add (free <ac->ac_g_ex.fe_len) judgment, and then delete
>     (free / fragments) >= ac->ac_g_ex.fe_len), because cr0 returns
>     true by default.
> 
> Signed-off-by: Chunguang Xu <brookxu@...cent.com>


Nice cleanup. This makes it less confusing :)

Logic looks fine to me.
Reviewed-by: Ritesh Harjani <riteshh@...ux.ibm.com>


> ---
>   fs/ext4/mballoc.c | 14 +++++++-------
>   1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 28a139f..4304113 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2119,13 +2119,11 @@ static bool ext4_mb_good_group(struct ext4_allocation_context *ac,
> 
>   	BUG_ON(cr < 0 || cr >= 4);
> 
> -	free = grp->bb_free;
> -	if (free == 0)
> -		return false;
> -	if (cr <= 2 && free < ac->ac_g_ex.fe_len)
> +	if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(grp)))
>   		return false;
> 
> -	if (unlikely(EXT4_MB_GRP_BBITMAP_CORRUPT(grp)))
> +	free = grp->bb_free;
> +	if (free == 0)
>   		return false;
> 
>   	fragments = grp->bb_fragments;
> @@ -2142,8 +2140,10 @@ static bool ext4_mb_good_group(struct ext4_allocation_context *ac,
>   		    ((group % flex_size) == 0))
>   			return false;
> 
> -		if ((ac->ac_2order > ac->ac_sb->s_blocksize_bits+1) ||
> -		    (free / fragments) >= ac->ac_g_ex.fe_len)
> +		if (free < ac->ac_g_ex.fe_len)
> +			return false;
> +
> +		if (ac->ac_2order > ac->ac_sb->s_blocksize_bits+1)
>   			return true;
> 
>   		if (grp->bb_largest_free_order < ac->ac_2order)
>