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Currently, CMA manages one range of physically contiguous memory. Creation of larger CMA areas with hugetlb_cma may run in to gaps in physical memory, so that they are not able to allocate that contiguous physical range from memblock when creating the CMA area. This can happen, for example, on an AMD system with > 1TB of memory, where there will be a gap just below the 1TB (40bit DMA) line. If you have set aside most of memory for potential hugetlb CMA allocation, cma_declare_contiguous_nid will fail. hugetlb_cma doesn't need the entire area to be one physically contiguous range. It just cares about being able to get physically contiguous chunks of a certain size (e.g. 1G), and it is fine to have the CMA area backed by multiple physical ranges, as long as it gets 1G contiguous allocations. Multi-range support is implemented by introducing an array of ranges, instead of just one big one. Each range has its own bitmap. Effectively, the allocate and release operations work as before, just per-range. So, instead of going through one large bitmap, they now go through a number of smaller ones. The maximum number of supported ranges is 8, as defined in CMA_MAX_RANGES. Since some current users of CMA expect a CMA area to just use one physically contiguous range, only allow for multiple ranges if a new interface, cma_declare_contiguous_nid_multi, is used. The other interfaces will work like before, creating only CMA areas with 1 range. cma_declare_contiguous_nid_multi works as follows, mimicking the default "bottom-up, above 4G" reservation approach: 0) Try cma_declare_contiguous_nid, which will use only one region. If this succeeds, return. This makes sure that for all the cases that currently work, the behavior remains unchanged even if the caller switches from cma_declare_contiguous_nid to cma_declare_contiguous_nid_multi. 1) Select the largest free memblock ranges above 4G, with a maximum number of CMA_MAX_RANGES. 2) If we did not find at most CMA_MAX_RANGES that add up to the total size requested, return -ENOMEM. 3) Sort the selected ranges by base address. 4) Reserve them bottom-up until we get what we wanted. Link: https://lkml.kernel.org/r/20250228182928.2645936-3-fvdl@google.com Signed-off-by: Frank van der Linden <fvdl@google.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Alexander Gordeev <agordeev@linux.ibm.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Dan Carpenter <dan.carpenter@linaro.org> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: David Hildenbrand <david@redhat.com> Cc: Heiko Carstens <hca@linux.ibm.com> Cc: Joao Martins <joao.m.martins@oracle.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Madhavan Srinivasan <maddy@linux.ibm.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Muchun Song <muchun.song@linux.dev> Cc: Oscar Salvador <osalvador@suse.de> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Roman Gushchin (Cruise) <roman.gushchin@linux.dev> Cc: Usama Arif <usamaarif642@gmail.com> Cc: Vasily Gorbik <gor@linux.ibm.com> Cc: Yu Zhao <yuzhao@google.com> Cc: Zi Yan <ziy@nvidia.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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=====================
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CMA Debugfs Interface
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=====================
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The CMA debugfs interface is useful to retrieve basic information out of the
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different CMA areas and to test allocation/release in each of the areas.
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Each CMA area represents a directory under <debugfs>/cma/, represented by
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its CMA name like below:
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<debugfs>/cma/<cma_name>
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The structure of the files created under that directory is as follows:
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- [RO] base_pfn: The base PFN (Page Frame Number) of the CMA area.
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This is the same as ranges/0/base_pfn.
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- [RO] count: Amount of memory in the CMA area.
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- [RO] order_per_bit: Order of pages represented by one bit.
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- [RO] bitmap: The bitmap of allocated pages in the area.
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This is the same as ranges/0/base_pfn.
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- [RO] ranges/N/base_pfn: The base PFN of contiguous range N
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in the CMA area.
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- [RO] ranges/N/bitmap: The bit map of allocated pages in
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range N in the CMA area.
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- [WO] alloc: Allocate N pages from that CMA area. For example::
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echo 5 > <debugfs>/cma/<cma_name>/alloc
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would try to allocate 5 pages from the 'cma_name' area.
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- [WO] free: Free N pages from that CMA area, similar to the above.
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