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

[Pratyush Yadav <pratyush@kernel.org>]

On Thu, Oct 29 2026, Mike Rapoport wrote:

> On Wed, Oct 28, 2026 at 03:03:16PM +0100, Pratyush Yadav wrote:
>> 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@amd.com>
>> > ---
>> >  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()?
>
> Actually, we shouldn't expose cma_init_reserved_mem() at all.
>
> The current callers of cma_init_reserved_mem() should be switched to
> cma_declare_contiguous_nid() that does sanity checks before calling
> cma_init_reserved_mem().

Fair enough.

On a side note, there have been some internal bug reports on CmaFree in
/proc/meminfo being large but CmaTotal being zero. That's because KHO
bootmem counts as CmaFree due to MIGRATE_CMA, but never declares a CMA
area. No real problem, just the accounting mismatch throws people off
when looking at things.

I played around with marking KHO bootmem as proper CMA areas, and I
prototyped something that used cma_init_reserved_mem(), but I can switch
that over to cma_declare_contiguous_nid().

Anyway, the hard part in those patches is declaring a CMA area _after_
KHO, since now that isn't fully free. I haven't solved that problem yet.
Maybe some day.

>
> This way the callers won't need to keep the checks for e.g
> CMA_MIN_ALIGNMENT and we'll have the clear way to differentiate areas with
> fixed based from allocated ones.
>  
>> With that fixed,
>> 
>> Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
>> 
>> >  	*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

-- 
Regards,
Pratyush Yadav