linux-kernel - Re: [PATCH v6 10/10] liveupdate: kho: allocate metadata directly from the buddy allocator

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Message-ID: <aPXtWDPGHA2kCg32@kernel.org>
Date: Mon, 20 Oct 2025 11:05:44 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: akpm@...ux-foundation.org, brauner@...nel.org, corbet@....net,
	graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org,
	masahiroy@...nel.org, ojeda@...nel.org, pratyush@...nel.org,
	rdunlap@...radead.org, tj@...nel.org, jasonmiu@...gle.com,
	dmatlack@...gle.com, skhawaja@...gle.com
Subject: Re: [PATCH v6 10/10] liveupdate: kho: allocate metadata directly
 from the buddy allocator

On Sat, Oct 18, 2025 at 01:17:56PM -0400, Pasha Tatashin wrote:
> KHO allocates metadata for its preserved memory map using the slab
> 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 slab.
> 
> Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> Reviewed-by: Pratyush Yadav <pratyush@...nel.org>
> ---
>  kernel/liveupdate/kexec_handover.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 7c8e89a6b953..92662739a3a2 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -132,6 +132,8 @@ static struct kho_out kho_out = {
>  	.finalized = false,
>  };
>  
> +DEFINE_FREE(kho_free_page, void *, free_page((unsigned long)_T))
> +

Just drop kho_ prefix and stick it into include/linux/gfp.h:

DEFINE_FREE(free_page, void *, if (_T) free_page((unsigned long)_T))

>  static void *xa_load_or_alloc(struct xarray *xa, unsigned long index)
>  {
>  	void *res = xa_load(xa, index);
> @@ -139,7 +141,7 @@ static void *xa_load_or_alloc(struct xarray *xa, unsigned long index)
>  	if (res)
>  		return res;
>  
> -	void *elm __free(kfree) = kzalloc(PAGE_SIZE, GFP_KERNEL);
> +	void *elm __free(kho_free_page) = (void *)get_zeroed_page(GFP_KERNEL);
>  
>  	if (!elm)
>  		return ERR_PTR(-ENOMEM);
> @@ -352,9 +354,9 @@ static_assert(sizeof(struct khoser_mem_chunk) == PAGE_SIZE);
>  static struct khoser_mem_chunk *new_chunk(struct khoser_mem_chunk *cur_chunk,
>  					  unsigned long order)
>  {
> -	struct khoser_mem_chunk *chunk __free(kfree) = NULL;
> +	struct khoser_mem_chunk *chunk __free(kho_free_page) = NULL;
>  
> -	chunk = kzalloc(PAGE_SIZE, GFP_KERNEL);
> +	chunk = (void *)get_zeroed_page(GFP_KERNEL);
>  	if (!chunk)
>  		return ERR_PTR(-ENOMEM);
>  
> -- 
> 2.51.0.915.g61a8936c21-goog
> 

-- 
Sincerely yours,
Mike.