linux-kernel - Re: [PATCH 3/9] maple_tree: Modify the allocation method of mtree_alloc

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Date:   Tue, 25 Apr 2023 12:08:15 -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 3/9] maple_tree: Modify the allocation method of
 mtree_alloc_range/rrange()

* Peng Zhang <zhangpeng.00@...edance.com> [230425 07:05]:
> Let mtree_alloc_range() and mtree_alloc_rrange() use mas_empty_area()
> and mas_empty_area_rev() respectively for allocation to reduce code
> redundancy. And after doing this, we don't need to maintain two logically
> identical codes to improve maintainability.
> 
> In fact, mtree_alloc_range/rrange() has some bugs. For example, when
> dealing with min equals to max (mas_empty_area/area_rev() has been fixed),
> the allocation will fail.
> There are still some other bugs in it, I saw it with my naked eyes, but
> I didn't test it, for example:
> When mtree_alloc_range()->mas_alloc()->mas_awalk(), we set mas.index = min,
> mas.last = max - size. However, mas_awalk() requires mas.index = min,
> mas.last = max, which may lead to allocation failures.

Please don't re-state code in your commit messages.

Try to focus on what you did, and not why.

ie: Aligned mtree_alloc_range() to use the same internal function as
mas_empty_area().

> 
> Right now no users are using these two functions so the bug won't trigger,
> but this might trigger in the future.
> 
> Also use mas_store_gfp() instead of mas_fill_gap() as I don't see any
> difference between them.

Yeah, evolution of the code converged on the same design.  Thanks for
seeing this.

> 
> After doing this, we no longer need the three functions
> mas_fill_gap(), mas_alloc(), and mas_rev_alloc().

Let's just drop mtree_alloc_range() and mtree_alloc_rrange() and
whatever else you found here.  They were planned to simplify the mmap
code allocations, but since there would need to be arch involvement
(coloring, etc) and alignment, etc; it is better to leave this job to
the mm code itself.

> 
> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
>  lib/maple_tree.c | 45 ++++++++++++---------------------------------
>  1 file changed, 12 insertions(+), 33 deletions(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index aa55c914818a0..294d4c8668323 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -6362,32 +6362,20 @@ int mtree_alloc_range(struct maple_tree *mt, unsigned long *startp,
>  {
>  	int ret = 0;
>  
> -	MA_STATE(mas, mt, min, max - size);
> +	MA_STATE(mas, mt, 0, 0);
>  	if (!mt_is_alloc(mt))
>  		return -EINVAL;
>  
>  	if (WARN_ON_ONCE(mt_is_reserved(entry)))
>  		return -EINVAL;
>  
> -	if (min > max)
> -		return -EINVAL;
> -
> -	if (max < size)
> -		return -EINVAL;
> -
> -	if (!size)
> -		return -EINVAL;
> -
>  	mtree_lock(mt);
> -retry:
> -	mas.offset = 0;
> -	mas.index = min;
> -	mas.last = max - size;
> -	ret = mas_alloc(&mas, entry, size, startp);
> -	if (mas_nomem(&mas, gfp))
> -		goto retry;
> -
> +	ret = mas_empty_area(&mas, min, max, size);
> +	if (!ret)
> +		ret = mas_store_gfp(&mas, entry, gfp);
>  	mtree_unlock(mt);
> +	if (!ret)
> +		*startp = mas.index;
>  	return ret;
>  }
>  EXPORT_SYMBOL(mtree_alloc_range);
> @@ -6398,29 +6386,20 @@ int mtree_alloc_rrange(struct maple_tree *mt, unsigned long *startp,
>  {
>  	int ret = 0;
>  
> -	MA_STATE(mas, mt, min, max - size);
> +	MA_STATE(mas, mt, 0, 0);
>  	if (!mt_is_alloc(mt))
>  		return -EINVAL;
>  
>  	if (WARN_ON_ONCE(mt_is_reserved(entry)))
>  		return -EINVAL;
>  
> -	if (min >= max)
> -		return -EINVAL;
> -
> -	if (max < size - 1)
> -		return -EINVAL;
> -
> -	if (!size)
> -		return -EINVAL;
> -
>  	mtree_lock(mt);
> -retry:
> -	ret = mas_rev_alloc(&mas, min, max, entry, size, startp);
> -	if (mas_nomem(&mas, gfp))
> -		goto retry;
> -
> +	ret = mas_empty_area_rev(&mas, min, max, size);
> +	if (!ret)
> +		ret = mas_store_gfp(&mas, entry, gfp);
>  	mtree_unlock(mt);
> +	if (!ret)
> +		*startp = mas.index;
>  	return ret;
>  }
>  EXPORT_SYMBOL(mtree_alloc_rrange);
> -- 
> 2.20.1
>