Re: [PATCH v7 1/2] kho: Adopt radix tree for preserved memory tracking

[Mike Rapoport <rppt@kernel.org>]

Hi Jason,

On Thu, Jan 15, 2026 at 07:44:31PM -0800, Jason Miu wrote:
> Introduce a radix tree implementation for tracking preserved memory
> pages and switch the KHO memory tracking mechanism to use it. This
> lays the groundwork for a stateless KHO implementation that eliminates
> the need for serialization and the associated "finalize" state.
> 
> This patch introduces the core radix tree data structures and
> constants to the KHO ABI. It adds the radix tree node and leaf
> structures, along with documentation for the radix tree key encoding
> scheme that combines a page's physical address and order.
> 
> To support broader use by other kernel subsystems, such as hugetlb
> preservation, the core radix tree manipulation functions are exported
> as a public API.
> 
> The xarray-based memory tracking is replaced with this new radix tree
> implementation. The core KHO preservation and unpreservation functions
> are wired up to use the radix tree helpers. On boot, the second kernel
> restores the preserved memory map by walking the radix tree whose root
> physical address is passed via the FDT.
> 
> The ABI `compatible` version is bumped to "kho-v2" to reflect the
> structural changes in the preserved memory map and sub-FDT property
> names. This includes renaming "fdt" to "preserved-data" to better
> reflect that preserved state may use formats other than FDT.
> 
> Signed-off-by: Jason Miu <jasonmiu@google.com>

...

> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 49bf2cecab12..06adaf56cd69 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -5,6 +5,7 @@
>   * Copyright (C) 2025 Microsoft Corporation, Mike Rapoport <rppt@kernel.org>
>   * Copyright (C) 2025 Google LLC, Changyuan Lyu <changyuanl@google.com>
>   * Copyright (C) 2025 Pasha Tatashin <pasha.tatashin@soleen.com>
> + * Copyright (C) 2025 Google LLC, Jason Miu <jasonmiu@google.com>

It's already 2026 ;-)

>   */
>  
>  #define pr_fmt(fmt) "KHO: " fmt

...

> +int kho_radix_add_page(struct kho_radix_tree *tree,
> +		       unsigned long pfn, unsigned int order)
> +{
> +	/* Newly allocated nodes for error cleanup */
> +	struct kho_radix_node *intermediate_nodes[KHO_TREE_MAX_DEPTH] = { 0 };
> +	unsigned long key = kho_radix_encode_key(PFN_PHYS(pfn), order);
> +	struct kho_radix_node *new_node, *anchor_node;
> +	struct kho_radix_node *node = tree->root;
> +	unsigned int i, idx, anchor_idx;
> +	struct kho_radix_leaf *leaf;
> +	int err = 0;
> +
> +	if (WARN_ON_ONCE(!tree->root))
> +		return -EINVAL;
> +
> +	might_sleep();
> +
> +	guard(mutex)(&tree->lock);
> +
> +	/* Go from high levels to low levels */
> +	for (i = KHO_TREE_MAX_DEPTH - 1; i > 0; i--) {
> +		idx = kho_radix_get_table_index(key, i);
> +
> +		if (node->table[idx]) {
> +			node = phys_to_virt(node->table[idx]);
> +			continue;
> +		}
> +
> +		/* Next node is empty, create a new node for it */
> +		new_node = (struct kho_radix_node *)get_zeroed_page(GFP_KERNEL);
> +		if (!new_node) {
> +			err = -ENOMEM;
> +			goto err_free_nodes;
> +		}
> +
> +		node->table[idx] = virt_to_phys(new_node);
> +
> +		/*
> +		 * Capture the node where the new branch starts for cleanup
> +		 * if allocation fails.
> +		 */
> +		if (!anchor_node) {

I think anchor_node should be initialized to NULL for this to work.

> +			anchor_node = node;
> +			anchor_idx = idx;
> +		}
> +		intermediate_nodes[i] = new_node;
> +
> +		node = new_node;
> +	}
> +
> +	/* Handle the leaf level bitmap (level 0) */
> +	idx = kho_radix_get_bitmap_index(key);
> +	leaf = (struct kho_radix_leaf *)node;
> +	__set_bit(idx, leaf->bitmap);
> +
> +	return 0;
> +
> +err_free_nodes:
> +	for (i = KHO_TREE_MAX_DEPTH - 1; i > 0; i--) {
> +		if (intermediate_nodes[i])
> +			free_page((unsigned long)intermediate_nodes[i]);
> +	}
> +	if (anchor_node)
> +		anchor_node->table[anchor_idx] = 0;
> +
> +	return err;
> +}
> +EXPORT_SYMBOL_GPL(kho_radix_add_page);

...

> +		if (WARN_ON(!node->table[idx]))
> +			return;
> +
> +		node = phys_to_virt((phys_addr_t)node->table[idx]);

No need for casting.

> +		shift = ((level - 1) * KHO_TABLE_SIZE_LOG2) +
> +			KHO_BITMAP_SIZE_LOG2;
> +		key = start | (i << shift);
> +
> +		node = phys_to_virt((phys_addr_t)root->table[i]);

Ditto.

> @@ -1466,12 +1489,6 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>  		goto out;
>  	}
>  
> -	mem_map_phys = kho_get_mem_map_phys(fdt);
> -	if (!mem_map_phys) {
> -		err = -ENOENT;
> -		goto out;
> -	}

I think we should keep the logic that skips scratch initialization if there
were no memory preservations, like Pasha implemented here:
https://lkml.kernel.org/r/20251223140140.2090337-1-pasha.tatashin@soleen.com
(commit e1c3bfd091f3 ("kho: validate preserved memory map during
population") in today's mm tree)

We just should update the validation to work with the radix tree.

>  	scratch = early_memremap(scratch_phys, scratch_len);
>  	if (!scratch) {
>  		pr_warn("setup: failed to memremap scratch (phys=0x%llx, len=%lld)\n",
> @@ -1512,7 +1529,6 @@ void __init kho_populate(phys_addr_t fdt_phys, u64 fdt_len,
>  
>  	kho_in.fdt_phys = fdt_phys;
>  	kho_in.scratch_phys = scratch_phys;
> -	kho_in.mem_map_phys = mem_map_phys;
>  	kho_scratch_cnt = scratch_cnt;
>  	pr_info("found kexec handover data.\n");
>  
> -- 
> 2.52.0.457.g6b5491de43-goog
> 

-- 
Sincerely yours,
Mike.