linux-kernel - Re: [PATCH] maple_tree: do not preallocate nodes for slot stores

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Date:   Tue, 12 Dec 2023 13:48:56 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        maple-tree@...ts.infradead.org, akpm@...ux-foundation.org,
        liam.howlett@...cle.com, zhangpeng.00@...edance.com
Subject: Re: [PATCH] maple_tree: do not preallocate nodes for slot stores

On 12/12/23 12:57 PM, Matthew Wilcox wrote:
> On Tue, Dec 12, 2023 at 11:46:40AM -0800, Sidhartha Kumar wrote:
>> +	/* Slot store, does not require additional nodes */
>> +	if ((node_size == mas->end) && ((!mt_in_rcu(mas->tree))
>> +		|| (wr_mas.offset_end - mas->offset == 1)))
>> +		return 0;
> 
> Should we refactor this into a mas_is_slot_store() predicate?
  yes, I think we should add helper functions to identify the different type of 
stores. Thanks for the pointers to code style this is what I think the slot 
store identifying helper function would look like:

static inline bool mas_wr_is_slot_store(struct ma_wr_state *wr_mas)
{
	struct ma_state *mas = wr_mas->mas;
	unsigned char node_size = mas_wr_new_end(wr_mas);

	if ((node_size == mas->end) &&
	    (!mt_in_rcu(mas->tree) || (wr_mas->offset_end - mas->offset == 1)))
		return true;

	return false;
}

thanks,
Sid
> A few coding-style problems with it as it's currently written:
> 
> 1. The indentation on the second line is wrong.  It makes the
> continuation of the condition look like part of the statement.  Use
> extra whitespace to indent.  eg:
> 
> 	if ((node_size == mas->end) && ((!mt_in_rcu(mas->tree))
> 			|| (wr_mas.offset_end - mas->offset == 1)))
> 		return 0;
> 
> 2. The operator goes last on the line, not at the beginning of the
> continuation line.  ie:
> 
> 	if ((node_size == mas->end) && ((!mt_in_rcu(mas->tree)) ||
> 			(wr_mas.offset_end - mas->offset == 1)))
> 		return 0;
> 
> 3. You don't need parens around the !mt_in_rcu(mas->tree).  There's
> no ambiguity to solve here:
> 
> 	if ((node_size == mas->end) && (!mt_in_rcu(mas->tree) ||
> 			(wr_mas.offset_end - mas->offset == 1)))
> 		return 0;
> 
> But I'd write it as:
> 
> 	if ((node_size == mas->end) &&
> 	    (!mt_in_rcu(mas->tree) || (wr_mas.offset_end - mas->offset == 1)))
> 		return 0;
> 
> because then the whitespace matches how you're supposed to parse the
> condition, and so the next person to read this code will have an easier
> time of it.
>