Re: [PATCH] cma: make number of CMA areas dynamic, remove CONFIG_CMA_AREAS

From: Mike Kravetz
Date: Wed Sep 16 2020 - 18:56:22 EST

On 9/16/20 2:14 AM, Song Bao Hua (Barry Song) wrote:
>>> -----Original Message-----
>>> From: Mike Kravetz [mailto:mike.kravetz@xxxxxxxxxx]
>>> Sent: Wednesday, September 16, 2020 8:57 AM
>>> To: linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>>> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-mips@xxxxxxxxxxxxxxx
>>> Cc: Roman Gushchin <guro@xxxxxx>; Song Bao Hua (Barry Song)
>>> <song.bao.hua@xxxxxxxxxxxxx>; Mike Rapoport <rppt@xxxxxxxxxx>; Joonsoo
>>> Kim <js1304@xxxxxxxxx>; Rik van Riel <riel@xxxxxxxxxxx>; Aslan Bakirov
>>> <aslan@xxxxxx>; Michal Hocko <mhocko@xxxxxxxxxx>; Andrew Morton
>>> <akpm@xxxxxxxxxxxxxxxxxxxx>; Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> Subject: [PATCH] cma: make number of CMA areas dynamic, remove
>>> The number of distinct CMA areas is limited by the constant
>>> CONFIG_CMA_AREAS. In most environments, this was set to a default
>>> value of 7. Not too long ago, support was added to allocate hugetlb
>>> gigantic pages from CMA. More recent changes to make
>> dma_alloc_coherent
>>> NUMA-aware on arm64 added more potential users of CMA areas. Along
>>> with the dma_alloc_coherent changes, the default value of CMA_AREAS
>>> was bumped up to 19 if NUMA is enabled.
>>> It seems that the number of CMA users is likely to grow. Instead of
>>> using a static array for cma areas, use a simple linked list. These
>>> areas are used before normal memory allocators, so use the memblock
>>> allocator.
>>> Acked-by: Roman Gushchin <guro@xxxxxx>
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>> rfc->v1
>>> - Made minor changes suggested by Song Bao Hua (Barry Song)
>>> - Removed check for late calls to cma_init_reserved_mem that was part
>>> of RFC.
>>> - Added ACK from Roman Gushchin
>>> - Still in need of arm testing
>> Unfortunately, the test result on my arm64 board is negative, Linux can't boot
>> after applying
>> this patch.
>> I guess we have to hold on this patch for a while till this is fixed. BTW, Mike, do
>> you have
>> a qemu-based arm64 numa system to debug? It is very easy to reproduce, we
>> don't need to
>> use hugetlb_cma and pernuma_cma. Just the default cma will make the boot
>> hang.
> Hi Mike,
> I spent some time on debugging the boot issue and sent a patch here:
> All details and knic oops can be found there.
> pls feel free to merge my patch into your v2 if you want. And we probably need ack from
> arm maintainers.
> Also, +Will,
> Hi Will, the whole story is that Mike tried to remove the cma array with CONFIG_CMA_AREAS
> and moved to use memblock_alloc() to allocate cma area, so that the number of cma areas
> could be dynamic. It turns out it causes a kernel panic on arm64 during system boot as the
> returned address from memblock_alloc is invalid before paging_init() is done on arm64.

Thank you!

Based on your analysis, I am concerned that other architectures may also
have issues.

I suggest we remove this patch from your tree. I will audit all architectures
which enable CMA and look for similar issues there. Will then merge Barry's
patch into a V2 with any other arch specific changes.
Mike Kravetz