Re: [Chapter One] THP zones: the use cases of policy zones

[Barry Song <21cnbao@gmail.com>]

> There are three types of zones:
> 1. The first four zones partition the physical address space of CPU
>   memory.
> 2. The device zone provides interoperability between CPU and device
>    memory.
> 3. The movable zone commonly represents a memory allocation policy.
> 
> +
> +static void __init find_virt_zone(unsigned long occupied, unsigned long *zone_pfn)
> +{
> +	int i, nid;
> +	unsigned long node_avg, remaining;

Hi Yu,

I discovered that CMA can be part of virtual zones. For example:

Node 0, zone  NoMerge
  pages free     35945
      nr_free_pages 35945
      ...
      nr_free_cma  8128
  pagesets

CMA used to be available for order-0 anonymous allocations, and
the Android kernel even prioritized it by commit[1]
"ANDROID: cma: redirect page allocation to CMA"

/*
 * Used during anonymous page fault handling.
 */
struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
						unsigned long vaddr)
{
	gfp_t flags = GFP_HIGHUSER_MOVABLE | __GFP_ZERO | __GFP_CMA;
	/*
	 * If the page is mapped with PROT_MTE, initialise the tags at the
	 * point of allocation and page zeroing as this is usually faster than
	 * separate DC ZVA and STGM.
	 */
	if (vma->vm_flags & VM_MTE)
		flags |= __GFP_ZEROTAGS;
	return vma_alloc_folio(flags, 0, vma, vaddr, false);
}

I wonder if cma is still available to order-0 when it is located in
the nomerge/nosplit zone.

And while dma_alloc_coherent() or similar APIs want to get contiguous
memory from cma, are they still as easy as before if cma is a part
of virt zones?

[1] https://android.googlesource.com/kernel/common/+/1c8aebe4c072bf18409cc78fc84407e24a437302

Thanks
Barry