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Date:   Tue, 18 Aug 2020 11:55:57 +0100
From:   Brian Starkey <brian.starkey@arm.com>
To:     Hyesoo Yu <hyesoo.yu@samsung.com>
Cc:     sumit.semwal@linaro.org, minchan@kernel.org,
        akpm@linux-foundation.org, iamjoonsoo.kim@lge.com,
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Subject: Re: [PATCH 0/3] Chunk Heap Support on DMA-HEAP
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Hi,

On Tue, Aug 18, 2020 at 05:04:12PM +0900, Hyesoo Yu wrote:
> These patch series to introduce a new dma heap, chunk heap.
> That heap is needed for special HW that requires bulk allocation of
> fixed high order pages. For example, 64MB dma-buf pages are made up
> to fixed order-4 pages * 1024.
> 
> The chunk heap uses alloc_pages_bulk to allocate high order page.
> https://lore.kernel.org/linux-mm/20200814173131.2803002-1-minchan@kernel.org
> 
> The chunk heap is registered by device tree with alignment and memory node
> of contiguous memory allocator(CMA). Alignment defines chunk page size.
> For example, alignment 0x1_0000 means chunk page size is 64KB.
> The phandle to memory node indicates contiguous memory allocator(CMA).
> If device node doesn't have cma, the registration of chunk heap fails.

This reminds me of an ion heap developed at Arm several years ago:
https://git.linaro.org/landing-teams/working/arm/kernel.git/tree/drivers/staging/android/ion/ion_compound_page.c

Some more descriptive text here:
https://github.com/ARM-software/CPA

It maintains a pool of high-order pages with a worker thread to
attempt compaction and allocation to keep the pool filled, with high
and low watermarks to trigger freeing/allocating of chunks.
It implements a shrinker to allow the system to reclaim the pool under
high memory pressure.

Is maintaining a pool something you considered? From the
alloc_pages_bulk thread it sounds like you want to allocate 300M at a
time, so I expect if you tuned the pool size to match that it could
work quite well.

That implementation isn't using a CMA region, but a similar approach
could definitely be applied.

Thanks,
-Brian

> 
> The patchset includes the following:
>  - export dma-heap API to register kernel module dma heap.
>  - add chunk heap implementation.
>  - document of device tree to register chunk heap
> 
> Hyesoo Yu (3):
>   dma-buf: add missing EXPORT_SYMBOL_GPL() for dma heaps
>   dma-buf: heaps: add chunk heap to dmabuf heaps
>   dma-heap: Devicetree binding for chunk heap
> 
>  .../devicetree/bindings/dma-buf/chunk_heap.yaml    |  46 +++++
>  drivers/dma-buf/dma-heap.c                         |   2 +
>  drivers/dma-buf/heaps/Kconfig                      |   9 +
>  drivers/dma-buf/heaps/Makefile                     |   1 +
>  drivers/dma-buf/heaps/chunk_heap.c                 | 222 +++++++++++++++++++++
>  drivers/dma-buf/heaps/heap-helpers.c               |   2 +
>  6 files changed, 282 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/dma-buf/chunk_heap.yaml
>  create mode 100644 drivers/dma-buf/heaps/chunk_heap.c
> 
> -- 
> 2.7.4
>