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Date:   Tue, 12 Dec 2023 15:27:18 -0500
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        maple-tree@lists.infradead.org, akpm@linux-foundation.org,
        willy@infradead.org, zhangpeng.00@bytedance.com
Subject: Re: [PATCH] maple_tree: do not preallocate nodes for slot stores
Message-ID: <20231212202718.mbv5tk5i3lmmc4ar@revolver>
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References: <20231212194640.217966-1-sidhartha.kumar@oracle.com>
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* Sidhartha Kumar <sidhartha.kumar@oracle.com> [231212 14:46]:
> mas_preallocate() defaults to requesting 1 node for preallocation and then
> ,depending on the type of store, will update the request variable. There
> isn't a check for a slot store type, so slot stores are preallocating the
> default 1 node. Slot stores do not require any additional nodes, so add a
> check for the slot store case that will bypass node_count_gfp(). Update
> the tests to reflect that slot stores do not require allocations.
> 
> User visible effects of this bug include increased memory usage from the
> unneeded node that was allocated.
> 
> Fixes: 0b8bb544b1a7 ("maple_tree: update mas_preallocate() testing")
> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
> This patch passes the maple tree test suite. A seperate patch will be sent
> for a 6.6 stable backport as the node_end field was moved from the
> ma_wr_state to the ma_state in a recent patch which is not in 6.6.
> 
> 
>  lib/maple_tree.c                 | 6 ++++++
>  tools/testing/radix-tree/maple.c | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index e6954fa75eb5..e4a39beb1018 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5475,6 +5475,12 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  
>  	mas_wr_end_piv(&wr_mas);
>  	node_size = mas_wr_new_end(&wr_mas);
> +
> +	/* 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;
> +
>  	if (node_size >= mt_slots[wr_mas.type]) {
>  		/* Split, worst case for now. */
>  		request = 1 + mas_mt_height(mas) * 2;
> diff --git a/tools/testing/radix-tree/maple.c b/tools/testing/radix-tree/maple.c
> index 687886cebd9d..f1caf4bcf937 100644
> --- a/tools/testing/radix-tree/maple.c
> +++ b/tools/testing/radix-tree/maple.c
> @@ -35545,7 +35545,7 @@ static noinline void __init check_prealloc(struct maple_tree *mt)
>  	MT_BUG_ON(mt, mas_preallocate(&mas, ptr, GFP_KERNEL) != 0);
>  	allocated = mas_allocated(&mas);
>  	height = mas_mt_height(&mas);
> -	MT_BUG_ON(mt, allocated != 1);
> +	MT_BUG_ON(mt, allocated != 0);
>  	mas_store_prealloc(&mas, ptr);
>  	MT_BUG_ON(mt, mas_allocated(&mas) != 0);
>  
> -- 
> 2.42.0
>