Re: [PATCH v3 06/11] mm/cma: Allow dynamically creating CMA areas

["T.J. Mercier" <tjmercier@google.com>]

On Wed, Jul 1, 2026 at 9:09 AM Thierry Reding <thierry.reding@kernel.org> wrote:
>
> From: Thierry Reding <treding@nvidia.com>
>
> There is no technical reason why there should be a limited number of CMA
> regions, so extract some code into helpers and use them to create extra
> functions (cma_create() and cma_free()) that allow creating and freeing,
> respectively, CMA regions dynamically at runtime.
>
> The static array of CMA areas cannot be replaced by dynamically created
> areas because for many of them, allocation must not fail and some cases
> may need to initialize them before the slab allocator is even available.
> To account for this, keep these "early" areas in a separate list and
> track the dynamic areas in a separate list.

Hi, It looks like you'll also need to update the CMA dma-buf heap's
add_cma_heaps init function so that it adds all the CMA areas, not
just the early ones.

>
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Changes in v3:
> - rebase on top of recent linux-next, update kernel/dma/contiguous.c
> - use kzalloc_obj() instead of kzalloc() with sizeof()
>
> Changes in v2:
> - rename fixed number of CMA areas to reflect their main use
> - account for pages in dynamically allocated regions
> ---
>  arch/arm/mm/dma-mapping.c |   2 +-
>  arch/s390/mm/init.c       |   2 +-
>  include/linux/cma.h       |   8 +-
>  kernel/dma/contiguous.c   |   2 +-
>  mm/cma.c                  | 187 +++++++++++++++++++++++++++++++++++++---------
>  mm/cma.h                  |   5 +-
>  6 files changed, 165 insertions(+), 41 deletions(-)
>
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index f9bc53b60f99..934952ab2102 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -254,7 +254,7 @@ struct dma_contig_early_reserve {
>         unsigned long size;
>  };
>
> -static struct dma_contig_early_reserve dma_mmu_remap[MAX_CMA_AREAS] __initdata;
> +static struct dma_contig_early_reserve dma_mmu_remap[MAX_EARLY_CMA_AREAS] __initdata;
>
>  static int dma_mmu_remap_num __initdata;
>
> diff --git a/arch/s390/mm/init.c b/arch/s390/mm/init.c
> index f07168a0d3dd..f8f78f1434ea 100644
> --- a/arch/s390/mm/init.c
> +++ b/arch/s390/mm/init.c
> @@ -241,7 +241,7 @@ static int s390_cma_mem_notifier(struct notifier_block *nb,
>         mem_data.start = arg->start_pfn << PAGE_SHIFT;
>         mem_data.end = mem_data.start + (arg->nr_pages << PAGE_SHIFT);
>         if (action == MEM_GOING_OFFLINE)
> -               rc = cma_for_each_area(s390_cma_check_range, &mem_data);
> +               rc = cma_for_each_early_area(s390_cma_check_range, &mem_data);
>         return notifier_from_errno(rc);
>  }
>
> diff --git a/include/linux/cma.h b/include/linux/cma.h
> index 8555d38a97b1..fb7a4923c3ba 100644
> --- a/include/linux/cma.h
> +++ b/include/linux/cma.h
> @@ -7,7 +7,7 @@
>  #include <linux/numa.h>
>
>  #ifdef CONFIG_CMA_AREAS
> -#define MAX_CMA_AREAS  CONFIG_CMA_AREAS
> +#define MAX_EARLY_CMA_AREAS    CONFIG_CMA_AREAS
>  #endif
>
>  #define CMA_MAX_NAME 64
> @@ -57,8 +57,14 @@ struct page *cma_alloc_frozen_compound(struct cma *cma, unsigned int order);
>  bool cma_release_frozen(struct cma *cma, const struct page *pages,
>                 unsigned long count);
>
> +extern int cma_for_each_early_area(int (*it)(struct cma *cma, void *data), void *data);
>  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);
> +
> +extern struct cma *cma_create(phys_addr_t base, phys_addr_t size,
> +                             unsigned int order_per_bit, const char *name);
> +extern void cma_free(struct cma *cma);
> +
>  #endif
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index f754079a287d..7975551f69b3 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -52,7 +52,7 @@
>  #define CMA_SIZE_MBYTES 0
>  #endif
>
> -static struct cma *dma_contiguous_areas[MAX_CMA_AREAS];
> +static struct cma *dma_contiguous_areas[MAX_EARLY_CMA_AREAS];
>  static unsigned int dma_contiguous_areas_num;
>
>  static int dma_contiguous_insert_area(struct cma *cma)
> diff --git a/mm/cma.c b/mm/cma.c
> index a13ce4999b39..f989e2e98594 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -34,7 +34,12 @@
>  #include "internal.h"
>  #include "cma.h"
>
> -struct cma cma_areas[MAX_CMA_AREAS];
> +static DEFINE_MUTEX(cma_lock);
> +
> +struct cma cma_early_areas[MAX_EARLY_CMA_AREAS];
> +unsigned int cma_early_area_count;
> +
> +static LIST_HEAD(cma_areas);
>  unsigned int cma_area_count;
>
>  phys_addr_t cma_get_base(const struct cma *cma)
> @@ -198,7 +203,6 @@ static void __init cma_activate_area(struct cma *cma)
>                                 free_reserved_page(pfn_to_page(pfn));
>                 }
>         }
> -       totalcma_pages -= cma->count;
>         cma->available_count = cma->count = 0;
>         pr_err("CMA area %s could not be activated\n", cma->name);
>  }
> @@ -207,8 +211,8 @@ static int __init cma_init_reserved_areas(void)
>  {
>         int i;
>
> -       for (i = 0; i < cma_area_count; i++)
> -               cma_activate_area(&cma_areas[i]);
> +       for (i = 0; i < cma_early_area_count; i++)
> +               cma_activate_area(&cma_early_areas[i]);
>
>         return 0;
>  }
> @@ -219,41 +223,77 @@ void __init cma_reserve_pages_on_error(struct cma *cma)
>         set_bit(CMA_RESERVE_PAGES_ON_ERROR, &cma->flags);
>  }
>
> +static void __init cma_init_area(struct cma *cma, const char *name,
> +                                phys_addr_t size, unsigned int order_per_bit)
> +{
> +       if (name)
> +               strscpy(cma->name, name);
> +       else
> +               snprintf(cma->name, CMA_MAX_NAME,  "cma%d\n", cma_area_count);
> +
> +       cma->available_count = cma->count = size >> PAGE_SHIFT;
> +       cma->order_per_bit = order_per_bit;
> +
> +       INIT_LIST_HEAD(&cma->node);
> +}
> +
>  static int __init cma_new_area(const char *name, phys_addr_t size,
>                                unsigned int order_per_bit,
>                                struct cma **res_cma)
>  {
>         struct cma *cma;
>
> -       if (cma_area_count == ARRAY_SIZE(cma_areas)) {
> +       if (cma_early_area_count == ARRAY_SIZE(cma_early_areas)) {
>                 pr_err("Not enough slots for CMA reserved regions!\n");
>                 return -ENOSPC;
>         }
>
> +       mutex_lock(&cma_lock);
> +
>         /*
>          * Each reserved area must be initialised later, when more kernel
>          * subsystems (like slab allocator) are available.
>          */
> -       cma = &cma_areas[cma_area_count];
> -       cma_area_count++;
> +       cma = &cma_early_areas[cma_early_area_count];
> +       cma_early_area_count++;
>
> -       if (name)
> -               strscpy(cma->name, name);
> -       else
> -               snprintf(cma->name, CMA_MAX_NAME,  "cma%d\n", cma_area_count);
> +       cma_init_area(cma, name, size, order_per_bit);
>
> -       cma->available_count = cma->count = size >> PAGE_SHIFT;
> -       cma->order_per_bit = order_per_bit;
> -       *res_cma = cma;
>         totalcma_pages += cma->count;
> +       *res_cma = cma;
> +
> +       mutex_unlock(&cma_lock);
>
>         return 0;
>  }
>
>  static void __init cma_drop_area(struct cma *cma)
>  {
> +       mutex_lock(&cma_lock);
>         totalcma_pages -= cma->count;
> -       cma_area_count--;
> +       cma_early_area_count--;
> +       mutex_unlock(&cma_lock);
> +}
> +
> +static int __init cma_check_memory(phys_addr_t base, phys_addr_t size)
> +{
> +       if (!size || !memblock_is_region_reserved(base, size))
> +               return -EINVAL;
> +
> +       /*
> +        * CMA uses CMA_MIN_ALIGNMENT_BYTES as alignment requirement which
> +        * needs pageblock_order to be initialized. Let's enforce it.
> +        */
> +       if (!pageblock_order) {
> +               pr_err("pageblock_order not yet initialized. Called during early boot?\n");
> +               return -EINVAL;
> +       }
> +
> +       /* ensure minimal alignment required by mm core */
> +       if (!IS_ALIGNED(base | size, CMA_MIN_ALIGNMENT_BYTES))
> +               return -EINVAL;
> +
> +       return 0;
>  }
>
>  /**
> @@ -276,22 +316,9 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
>         struct cma *cma;
>         int ret;
>
> -       /* Sanity checks */
> -       if (!size || !memblock_is_region_reserved(base, size))
> -               return -EINVAL;
> -
> -       /*
> -        * CMA uses CMA_MIN_ALIGNMENT_BYTES as alignment requirement which
> -        * needs pageblock_order to be initialized. Let's enforce it.
> -        */
> -       if (!pageblock_order) {
> -               pr_err("pageblock_order not yet initialized. Called during early boot?\n");
> -               return -EINVAL;
> -       }
> -
> -       /* ensure minimal alignment required by mm core */
> -       if (!IS_ALIGNED(base | size, CMA_MIN_ALIGNMENT_BYTES))
> -               return -EINVAL;
> +       ret = cma_check_memory(base, size);
> +       if (ret < 0)
> +               return ret;
>
>         ret = cma_new_area(name, size, order_per_bit, &cma);
>         if (ret != 0)
> @@ -444,7 +471,7 @@ static int __init __cma_declare_contiguous_nid(phys_addr_t *basep,
>         pr_debug("%s(size %pa, base %pa, limit %pa alignment %pa)\n",
>                 __func__, &size, &base, &limit, &alignment);
>
> -       if (cma_area_count == ARRAY_SIZE(cma_areas)) {
> +       if (cma_early_area_count == ARRAY_SIZE(cma_early_areas)) {
>                 pr_err("Not enough slots for CMA reserved regions!\n");
>                 return -ENOSPC;
>         }
> @@ -1051,12 +1078,12 @@ bool cma_release_frozen(struct cma *cma, const struct page *pages,
>         return true;
>  }
>
> -int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
> +int cma_for_each_early_area(int (*it)(struct cma *cma, void *data), void *data)
>  {
>         int i;
>
> -       for (i = 0; i < cma_area_count; i++) {
> -               int ret = it(&cma_areas[i], data);
> +       for (i = 0; i < cma_early_area_count; i++) {
> +               int ret = it(&cma_early_areas[i], data);
>
>                 if (ret)
>                         return ret;
> @@ -1065,6 +1092,25 @@ int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
>         return 0;
>  }
>
> +int cma_for_each_area(int (*it)(struct cma *cma, void *data), void *data)
> +{
> +       struct cma *cma;
> +
> +       mutex_lock(&cma_lock);
> +
> +       list_for_each_entry(cma, &cma_areas, node) {
> +               int ret = it(cma, data);
> +
> +               if (ret) {
> +                       mutex_unlock(&cma_lock);
> +                       return ret;
> +               }
> +       }
> +
> +       mutex_unlock(&cma_lock);
> +       return 0;
> +}
> +
>  bool cma_intersects(struct cma *cma, unsigned long start, unsigned long end)
>  {
>         int r;
> @@ -1147,3 +1193,74 @@ void __init *cma_reserve_early(struct cma *cma, unsigned long size)
>
>         return ret;
>  }
> +
> +struct cma *__init cma_create(phys_addr_t base, phys_addr_t size,
> +                             unsigned int order_per_bit, const char *name)
> +{
> +       struct cma *cma;
> +       int ret;
> +
> +       ret = cma_check_memory(base, size);
> +       if (ret < 0)
> +               return ERR_PTR(ret);
> +
> +       cma = kzalloc_obj(*cma, GFP_KERNEL);
> +       if (!cma)
> +               return ERR_PTR(-ENOMEM);
> +
> +       cma_init_area(cma, name, size, order_per_bit);
> +       cma->ranges[0].base_pfn = PFN_DOWN(base);
> +       cma->ranges[0].early_pfn = PFN_DOWN(base);
> +       cma->ranges[0].count = cma->count;
> +       cma->nranges = 1;
> +
> +       cma_activate_area(cma);
> +
> +       mutex_lock(&cma_lock);
> +       list_add_tail(&cma->node, &cma_areas);
> +       totalcma_pages += cma->count;
> +       cma_area_count++;
> +       mutex_unlock(&cma_lock);
> +
> +       return cma;
> +}
> +
> +void cma_free(struct cma *cma)
> +{
> +       unsigned int i;
> +
> +       /*
> +        * Safety check to prevent a CMA with active allocations from being
> +        * released.
> +        */
> +       for (i = 0; i < cma->nranges; i++) {
> +               unsigned long nbits = cma_bitmap_maxno(cma, &cma->ranges[i]);
> +
> +               if (!bitmap_empty(cma->ranges[i].bitmap, nbits)) {
> +                       WARN(1, "%s: range %u not empty\n", cma->name, i);
> +                       return;
> +               }
> +       }
> +
> +       /* free reserved pages and the bitmap */
> +       for (i = 0; i < cma->nranges; i++) {
> +               struct cma_memrange *cmr = &cma->ranges[i];
> +               unsigned long end_pfn, pfn;
> +
> +               end_pfn = cmr->base_pfn + cmr->count;
> +               for (pfn = cmr->base_pfn; pfn < end_pfn; pfn++)
> +                       free_reserved_page(pfn_to_page(pfn));
> +
> +               bitmap_free(cmr->bitmap);
> +       }
> +
> +       mutex_destroy(&cma->alloc_mutex);
> +
> +       mutex_lock(&cma_lock);
> +       totalcma_pages -= cma->count;
> +       list_del(&cma->node);
> +       cma_area_count--;
> +       mutex_unlock(&cma_lock);
> +
> +       kfree(cma);
> +}
> diff --git a/mm/cma.h b/mm/cma.h
> index c70180c36559..ae4db9819e38 100644
> --- a/mm/cma.h
> +++ b/mm/cma.h
> @@ -41,6 +41,7 @@ struct cma {
>         unsigned long   available_count;
>         unsigned int order_per_bit; /* Order of pages represented by one bit */
>         spinlock_t      lock;
> +       struct list_head node;
>         struct mutex alloc_mutex;
>  #ifdef CONFIG_CMA_DEBUGFS
>         struct hlist_head mem_head;
> @@ -71,8 +72,8 @@ enum cma_flags {
>         CMA_ACTIVATED,
>  };
>
> -extern struct cma cma_areas[MAX_CMA_AREAS];
> -extern unsigned int cma_area_count;
> +extern struct cma cma_early_areas[MAX_EARLY_CMA_AREAS];
> +extern unsigned int cma_early_area_count;
>
>  static inline unsigned long cma_bitmap_maxno(struct cma *cma,
>                 struct cma_memrange *cmr)
>
> --
> 2.54.0
>