Re: [PATCH 2/3] maple_tree: Fix MA_STATE_PREALLOC flag in mas_preallocate()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jun 16, 2025 at 02:45:20PM -0400, Liam R. Howlett wrote:
> Temporarily clear the preallocation flag when explicitly requesting
> allocations.  Pre-existing allocations are already counted against the
> request through mas_node_count_gfp(), but the allocations will not
> happen if the MA_STATE_PREALLOC flag is set.  This flag is meant to
> avoid re-allocating in bulk allocation mode, and to detect issues with
> preallocation calculations.
>
> The MA_STATE_PREALLOC flag should also always be set on zero allocations
> so that detection of underflow allocations will print a WARN_ON() during
> consumption.
>
> User visible effect of this flaw is a WARN_ON() followed by a null
> pointer dereference when subsequent requests for larger number of nodes
> is ignored, such as the vma merge retry in mmap_region() caused by
> drivers altering the vma flags (which happens in v6.6, at least)

Oh interesting, this was a deep one (Lovecraftian pun fully intended)!

>
> Reported-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> Reported-by: Hailong Liu <hailong.liu@oppo.com>
> Fixes: 54a611b605901 ("Maple Tree: add new data structure")
> Link: https://lore.kernel.org/all/1652f7eb-a51b-4fee-8058-c73af63bacd1@oppo.com/
> Link: https://lore.kernel.org/all/20250428184058.1416274-1-Liam.Howlett@oracle.com/
> Link: https://lore.kernel.org/all/20250429014754.1479118-1-Liam.Howlett@oracle.com/
> Cc: stable@kernel.org
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Hailong Liu <hailong.liu@oppo.com>
> Cc: zhangpeng.00@bytedance.com <zhangpeng.00@bytedance.com>
> Cc: Steve Kang <Steve.Kang@unisoc.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>

This looks reasonable, but because I am not quite au fait with all the details
on a deeper level:

Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  lib/maple_tree.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 7144dbbc34813..54a09c26edf96 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5528,8 +5528,9 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  	mas->store_type = mas_wr_store_type(&wr_mas);
>  	request = mas_prealloc_calc(&wr_mas, entry);
>  	if (!request)
> -		return ret;
> +		goto set_flag;
>
> +	mas->mas_flags &= ~MA_STATE_PREALLOC;
>  	mas_node_count_gfp(mas, request, gfp);
>  	if (mas_is_err(mas)) {
>  		mas_set_alloc_req(mas, 0);
> @@ -5539,6 +5540,7 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  		return ret;
>  	}
>
> +set_flag:
>  	mas->mas_flags |= MA_STATE_PREALLOC;
>  	return ret;
>  }
> --
> 2.47.2
>