Re: [RFC PATCH 1/2] kexec: do not place the next kernel in a fixed-address CMA area

[Pratyush Yadav]

On Wed, Oct 14 2026, Radhey Shyam Pandey wrote:

> kexec_alloc_contig() picks the destination of the next kernel out of the
> default CMA area, and the next kernel then runs from there in place
> instead of being copied out. That is only safe if the area will not exist
> at the same address in the kernel being loaded.
>
> For an area whose base came from the device tree, it will. The loaded
> kernel parses the same description, declares the range it is executing
> from as a shared-dma-pool, and cma_activate_area() releases the pages
> holding its own image to the page allocator.
>
> Distinguish the two cases. An area placed by cma_declare_contiguous_nid()
> without a fixed base lands wherever this kernel found room, and the next
> kernel repeats that search against its own memblock state, so it cannot
> collide. Record the difference as CMA_FIXED_BASE and refuse to use such
> an area for the payload, which leaves kexec_locate_mem_hole() to fall
> back to the memblock hole search as it did before CMA placement existed.
>
> This is what makes the KEXEC_FILE_NO_CMA flag unnecessary on platforms
> that describe their CMA pools in the device tree.
>
> Tested on a AMD Versal AI Edge Series Gen 2 VEK385 Evaluation Kit whose
> device tree declares three CMA pools, the first of which is
> linux,cma-default:
>
>   cma_reserved@800000000     4 GiB at 0x800000000    (default)
>   cma_reserved@50000000000   6 GiB at 0x50000000000
>   cma_reserved@60000000000   4 GiB at 0x60000000000
>
> All three have a fixed base, so kexec_alloc_contig() declines the default
> area and kexec_locate_mem_hole() falls through to the memblock search,
> which places the Image segment at 0x9c00000 size 0x1de0000. The kernel
> that boots from it reports its image as [0x09c00000-0x0b9e0000] and
> 14680064K cma-reserved, i.e. all three pools still present and none of
> them overlapping the running kernel.
>
> Fixes: 07d24902977e ("kexec: enable CMA based contiguous allocation")
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xxxxxxx>
> ---
>  include/linux/cma.h | 10 ++++++++++
>  kernel/kexec_file.c | 12 ++++++++++++
>  mm/cma.c            | 16 ++++++++++++++++
>  mm/cma.h            |  2 ++
>  4 files changed, 40 insertions(+)
>
> diff --git a/include/linux/cma.h b/include/linux/cma.h
> index 8555d38a97b1..2fe66377e5d3 100644
> --- a/include/linux/cma.h
> +++ b/include/linux/cma.h
> @@ -61,4 +61,14 @@ extern int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
>  extern bool cma_intersects(struct cma *cma, unsigned long start, unsigned long end);
>  
>  extern void cma_reserve_pages_on_error(struct cma *cma);
> +
> +#ifdef CONFIG_CMA
> +bool cma_has_fixed_base(const struct cma *cma);
> +#else
> +static inline bool cma_has_fixed_base(const struct cma *cma)
> +{
> +	return false;
> +}
> +#endif
> +
>  #endif
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index b668ce8d68ee..10e770bcee0b 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -28,6 +28,7 @@
>  #include <linux/vmalloc.h>
>  #include <linux/dma-map-ops.h>
>  #include <linux/kexec_handover.h>
> +#include <linux/cma.h>
>  #include "kexec_internal.h"
>  
>  #ifdef CONFIG_KEXEC_SIG
> @@ -691,6 +692,7 @@ static int kexec_walk_resources(struct kexec_buf *kbuf,
>  static int kexec_alloc_contig(struct kexec_buf *kbuf)
>  {
>  	size_t nr_pages = kbuf->memsz >> PAGE_SHIFT;
> +	struct cma *cma;
>  	unsigned long mem;
>  	struct page *p;
>  
> @@ -702,6 +704,16 @@ static int kexec_alloc_contig(struct kexec_buf *kbuf)
>  	if (kbuf->image->type == KEXEC_TYPE_CRASH)
>  		return -EPERM;
>  
> +	/*
> +	 * The next kernel runs from here rather than being copied out, so it
> +	 * would declare this area again from the same description and release
> +	 * the pages holding its own image to the page allocator. Only an area
> +	 * this kernel placed itself is safe to use.
> +	 */
> +	cma = dev_get_cma_area(NULL);
> +	if (!cma || cma_has_fixed_base(cma))
> +		return -EOPNOTSUPP;
> +
>  	p = dma_alloc_from_contiguous(NULL, nr_pages, get_order(kbuf->buf_align), true);
>  	if (!p)
>  		return -ENOMEM;
> diff --git a/mm/cma.c b/mm/cma.c
> index a10ea37a261d..1f4d3d48a3bb 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -55,6 +55,19 @@ const char *cma_get_name(const struct cma *cma)
>  }
>  EXPORT_SYMBOL_GPL(cma_get_name);
>  
> +/**
> + * cma_has_fixed_base() - was the area placed at an address it did not choose?
> + * @cma: Contiguous memory region
> + *
> + * Such an area is described outside the kernel, typically by a device tree
> + * reserved-memory node, and so lands at the same address in every kernel
> + * booted with that description.
> + */
> +bool cma_has_fixed_base(const struct cma *cma)
> +{
> +	return test_bit(CMA_FIXED_BASE, &cma->flags);
> +}
> +
>  static unsigned long cma_bitmap_aligned_mask(const struct cma *cma,
>  					     unsigned int align_order)
>  {
> @@ -304,6 +317,7 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
>  	cma->ranges[0].count = cma->count;
>  	cma->nranges = 1;
>  	cma->nid = NUMA_NO_NODE;
> +	set_bit(CMA_FIXED_BASE, &cma->flags);
>  
>  	*res_cma = cma;
>  
> @@ -517,6 +531,8 @@ static int __init __cma_declare_contiguous_nid(phys_addr_t *basep,
>  	}
>  
>  	(*res_cma)->nid = nid;
> +	if (!fixed)
> +		clear_bit(CMA_FIXED_BASE, &(*res_cma)->flags);

Nit: This reads a bit odd. cma_init_reserved_mem() sets the fixed base
and then you clear it later. Wouldn't it be better to just pass the
fixed boolean to cma_init_reserved_mem()?

With that fixed,

Reviewed-by: Pratyush Yadav (Google) <pratyush@xxxxxxxxxx>

>  	*basep = base;
>  
>  	return 0;
> diff --git a/mm/cma.h b/mm/cma.h
> index 68e574b0f951..1bab24033284 100644
> --- a/mm/cma.h
> +++ b/mm/cma.h
> @@ -80,6 +80,8 @@ enum cma_flags {
>  	CMA_ZONES_VALID,
>  	CMA_ZONES_INVALID,
>  	CMA_ACTIVATED,
> +	/* Base address was dictated by the caller, not picked by CMA. */
> +	CMA_FIXED_BASE,
>  };
>  
>  extern struct cma cma_areas[MAX_CMA_AREAS];

-- 
Regards,
Pratyush Yadav