linux-kernel - Re: [PATCH v2 07/10] maple_tree: Add comments and some minor cleanups to mas_wr

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Message-ID: <20230517193353.xwd6vztfjqxesyl6@revolver>
Date:   Wed, 17 May 2023 15:33:53 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Peng Zhang <zhangpeng.00@...edance.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, maple-tree@...ts.infradead.org
Subject: Re: [PATCH v2 07/10] maple_tree: Add comments and some minor
 cleanups to mas_wr_append()

* Peng Zhang <zhangpeng.00@...edance.com> [230517 04:59]:
> Add comment for mas_wr_append(), move mas_update_gap() into
> mas_wr_append(), and other cleanups to make mas_wr_modify() cleaner.
> 
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
>  lib/maple_tree.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 3b9d227f3d7d..bbe4c6f2858c 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -4308,6 +4308,12 @@ static inline unsigned char mas_wr_new_end(struct ma_wr_state *wr_mas)
>  	return new_end;
>  }
>  
> +/*
> + * mas_wr_append: Attempt to append
> + * @wr_mas: the maple write state
> + *
> + * Return: True if appended, false otherwise
> + */
>  static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
>  {
>  	unsigned char end = wr_mas->node_end;
> @@ -4315,7 +4321,11 @@ static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
>  	struct ma_state *mas = wr_mas->mas;
>  	unsigned char node_pivots = mt_pivots[wr_mas->type];
>  
> +	if (!(mas->offset == wr_mas->node_end))
> +		return false;
> +
>  	if ((mas->index != wr_mas->r_min) && (mas->last == wr_mas->r_max)) {
> +		/* Append to end of range */
>  		if (new_end < node_pivots)
>  			wr_mas->pivots[new_end] = wr_mas->pivots[end];
>  
> @@ -4323,13 +4333,10 @@ static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
>  			ma_set_meta(wr_mas->node, maple_leaf_64, 0, new_end);
>  
>  		rcu_assign_pointer(wr_mas->slots[new_end], wr_mas->entry);
> -		mas->offset = new_end;
>  		wr_mas->pivots[end] = mas->index - 1;
> -
> -		return true;
> -	}
> -
> -	if ((mas->index == wr_mas->r_min) && (mas->last < wr_mas->r_max)) {
> +		mas->offset = new_end;
> +	} else if ((mas->index == wr_mas->r_min) && (mas->last < wr_mas->r_max)) {
> +		/* Append to start of range */
>  		if (new_end < node_pivots)
>  			wr_mas->pivots[new_end] = wr_mas->pivots[end];
>  
> @@ -4339,10 +4346,13 @@ static inline bool mas_wr_append(struct ma_wr_state *wr_mas)
>  
>  		wr_mas->pivots[end] = mas->last;
>  		rcu_assign_pointer(wr_mas->slots[end], wr_mas->entry);
> -		return true;
> +	} else {
> +		return false;

I don't think we can get here with your changes, what do you think?  If
so, we can move the metadata setting to the outside of the if/else.  I
checked by adding a BUG_ON() and the test code never gets here.

My thinking is, we know offset == node_end and new_end == node_end + 1..
so we must be inserting into the last slot and only adding one entry.

>  	}
>  
> -	return false;
> +	if (!wr_mas->content || !wr_mas->entry)
> +		mas_update_gap(mas);
> +	return  true;
>  }
>  
>  /*
> @@ -4382,12 +4392,9 @@ static inline void mas_wr_modify(struct ma_wr_state *wr_mas)
>  	if (new_end >= mt_slots[wr_mas->type])
>  		goto slow_path;
>  
> -	if (wr_mas->entry && (wr_mas->node_end < mt_slots[wr_mas->type] - 1) &&
> -	    (mas->offset == wr_mas->node_end) && mas_wr_append(wr_mas)) {
> -		if (!wr_mas->content || !wr_mas->entry)
> -			mas_update_gap(mas);
> +	/* Attempt to append */
> +	if (new_end == wr_mas->node_end + 1 && mas_wr_append(wr_mas))
>  		return;
> -	}
>  
>  	if ((wr_mas->offset_end - mas->offset <= 1) && mas_wr_slot_store(wr_mas))
>  		return;
> -- 
> 2.20.1
>