linux-kernel - Re: [PATCH 6.6.y] maple_tree: do not preallocate nodes for slot stores

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Date:   Wed, 13 Dec 2023 12:54:37 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        maple-tree@...ts.infradead.org
Cc:     akpm@...ux-foundation.org, willy@...radead.org,
        liam.howlett@...cle.com, zhangpeng.00@...edance.com,
        stable@...r.kernel.org
Subject: Re: [PATCH 6.6.y] maple_tree: do not preallocate nodes for slot
 stores

On 12/12/23 11:52 AM, Sidhartha Kumar wrote:
> 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")
> Cc: <stable@...r.kernel.org> # 6.6+
> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
> ---
> This is a modified backport as the patch to fix this in upstream does not
> apply to 6.6 because the node_end field was moved from the ma_wr_state to
> the ma_state after 6.6.
  Please disregard this patch, the upstream fix[1] does actually apply correctly 
to the 6.6 stable branch.

Sorry for the noise,
Sid

[1] 
https://lore.kernel.org/linux-mm/20231213205058.386589-1-sidhartha.kumar@oracle.com/T/#u



>   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 bb24d84a4922f..5950d0c0e0f69 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5501,6 +5501,12 @@ 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 == wr_mas.node_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 e5da1cad70baf..76a8990bb14e8 100644
> --- a/tools/testing/radix-tree/maple.c
> +++ b/tools/testing/radix-tree/maple.c
> @@ -35538,7 +35538,7 @@ 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);
>