On Wed, Oct 15, 2025 at 01:31:21AM -0400, Pasha Tatashin wrote:
> KHO allocates metadata for its preserved memory map using the SLUB
> allocator via kzalloc(). This metadata is temporary and is used by the
> next kernel during early boot to find preserved memory.
>
> A problem arises when KFENCE is enabled. kzalloc() calls can be
> randomly intercepted by kfence_alloc(), which services the allocation
> from a dedicated KFENCE memory pool. This pool is allocated early in
> boot via memblock.
>
> When booting via KHO, the memblock allocator is restricted to a "scratch
> area", forcing the KFENCE pool to be allocated within it. This creates a
> conflict, as the scratch area is expected to be ephemeral and
> overwriteable by a subsequent kexec. If KHO metadata is placed in this
> KFENCE pool, it leads to memory corruption when the next kernel is
> loaded.
>
> To fix this, modify KHO to allocate its metadata directly from the buddy
> allocator instead of SLUB.
>
> As part of this change, the metadata bitmap size is increased from 512
> bytes to PAGE_SIZE to align with the page-based allocations from the
> buddy system.
>
> Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
> Signed-off-by: Pasha Tatashin <[email protected]>
> ---
> kernel/liveupdate/kexec_handover.c | 23 +++++++++++++----------
> 1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c
> b/kernel/liveupdate/kexec_handover.c
> index ef1e6f7a234b..519de6d68b27 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -66,10 +66,10 @@ early_param("kho", kho_parse_enable);
> * Keep track of memory that is to be preserved across KHO.
> *
> * The serializing side uses two levels of xarrays to manage chunks of
> per-order
> - * 512 byte bitmaps. For instance if PAGE_SIZE = 4096, the entire 1G order
> of a
> - * 1TB system would fit inside a single 512 byte bitmap. For order 0
> allocations
> - * each bitmap will cover 16M of address space. Thus, for 16G of memory at
> most
> - * 512K of bitmap memory will be needed for order 0.
> + * PAGE_SIZE byte bitmaps. For instance if PAGE_SIZE = 4096, the entire 1G
> order
> + * of a 8TB system would fit inside a single 4096 byte bitmap. For order 0
> + * allocations each bitmap will cover 128M of address space. Thus, for 16G of
> + * memory at most 512K of bitmap memory will be needed for order 0.
> *
> * This approach is fully incremental, as the serialization progresses folios
> * can continue be aggregated to the tracker. The final step, immediately
> prior
> @@ -77,7 +77,7 @@ early_param("kho", kho_parse_enable);
> * successor kernel to parse.
> */
>
> -#define PRESERVE_BITS (512 * 8)
> +#define PRESERVE_BITS (PAGE_SIZE * 8)
>
> struct kho_mem_phys_bits {
> DECLARE_BITMAP(preserve, PRESERVE_BITS);
> @@ -131,18 +131,21 @@ static struct kho_out kho_out = {
>
> static void *xa_load_or_alloc(struct xarray *xa, unsigned long index, size_t
> sz)
The name 'xa_load_or_alloc' is confusing now that we only use this function
to allocate bitmaps. I think it should be renamed to reflect that and it's
return type should be 'struct kho_mem_phys_bits'. Then it wouldn't need sz
parameter and the size calculations below become redundant.
> {
> + unsigned int order;
> void *elm, *res;
>
> elm = xa_load(xa, index);
> if (elm)
> return elm;
>
> - elm = kzalloc(sz, GFP_KERNEL);
> + order = get_order(sz);
> + elm = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, order);
> if (!elm)
> return ERR_PTR(-ENOMEM);
>
> - if (WARN_ON(kho_scratch_overlap(virt_to_phys(elm), sz))) {
> - kfree(elm);
> + if (WARN_ON(kho_scratch_overlap(virt_to_phys(elm),
> + PAGE_SIZE << order))) {
> + free_pages((unsigned long)elm, order);
> return ERR_PTR(-EINVAL);
> }
>
--
Sincerely yours,
Mike.
