[RFC PATCH v6.6] maple_tree: Fix MA_STATE_PREALLOC flag in mas_preallocate()

[Hailong Liu]

On 4/29/2025 9:47 AM, 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.
> 
> Reported-by: Zhaoyang Huang <zhaoyang.huang at unisoc.com>
> Reported-by: Hailong Liu <hailong.liu at 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/
> Cc: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
> Cc: Suren Baghdasaryan <surenb at google.com>
> Cc: Hailong Liu <hailong.liu at oppo.com>
> Cc: zhangpeng.00 at bytedance.com <zhangpeng.00 at bytedance.com>
> Cc: Steve Kang <Steve.Kang at unisoc.com>
> Cc: Matthew Wilcox <willy at infradead.org>
> Cc: Sidhartha Kumar <sidhartha.kumar at oracle.com>
> Signed-off-by: Liam R. Howlett <Liam.Howlett at oracle.com>
> ---
>  lib/maple_tree.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> Only the MA_STATE_PREALLOC flag needs to be fixed to avoid the vma
> iterator issue.  Doing the minimum change here mitigates risk in the
> stable kernels.
> 
> If this fixes the issue, I'll resend without the RFC and add Stable to
> the Cc list.
> 
> Thanks again, Hailong for the work on this issue.  Your testcase helped
> me narrow the bug down in the end.  I appreciate all the work and
> support provided by the Android partners, especially the involvement on
> the mailing list!
That's my pleasure. I also learned a lot from you :)
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index 4eda949063602..d9975b870dadc 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5508,7 +5508,7 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  	/* At this point, we are at the leaf node that needs to be altered. */
>  	/* Exact fit, no nodes needed. */
>  	if (wr_mas.r_min == mas->index && wr_mas.r_max == mas->last)
> -		return 0;
> +		goto ask_zero;
>  
>  	mas_wr_end_piv(&wr_mas);
>  	node_size = mas_wr_new_end(&wr_mas);
> @@ -5517,10 +5517,11 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  	if (node_size == wr_mas.node_end) {
>  		/* reuse node */
>  		if (!mt_in_rcu(mas->tree))
> -			return 0;
> +			goto ask_zero;
> +
>  		/* shifting boundary */
>  		if (wr_mas.offset_end - mas->offset == 1)
> -			return 0;
> +			goto ask_zero;
>  	}
>  
>  	if (node_size >= mt_slots[wr_mas.type]) {
> @@ -5539,10 +5540,13 @@ int mas_preallocate(struct ma_state *mas, void *entry, gfp_t gfp)
>  
>  	/* node store, slot store needs one node */
>  ask_now:
> +	mas->mas_flags &= ~MA_STATE_PREALLOC;
>  	mas_node_count_gfp(mas, request, gfp);
> -	mas->mas_flags |= MA_STATE_PREALLOC;
> -	if (likely(!mas_is_err(mas)))
> +	if (likely(!mas_is_err(mas))) {
> +ask_zero:
> +		mas->mas_flags |= MA_STATE_PREALLOC;
>  		return 0;
> +	}
>  
>  	mas_set_alloc_req(mas, 0);
>  	ret = xa_err(mas->node);
Passed local test, planned to aging test. 

Brs,
Hailong.