[RFC PATCH 0/3] Use pageblock_order for cma and alloc_contig_range alignment.

[Zi Yan]

On 19 Nov 2021, at 7:33, Vlastimil Babka wrote:

> On 11/15/21 20:37, Zi Yan wrote:
>> From: Zi Yan <ziy at nvidia.com>
>>
>> Hi David,
>>
>> You suggested to make alloc_contig_range() deal with pageblock_order instead of
>> MAX_ORDER - 1 and get rid of MAX_ORDER - 1 dependency in virtio_mem[1]. This
>> patchset is my attempt to achieve that. Please take a look and let me know if
>> I am doing it correctly or not.
>>
>> From what my understanding, cma required alignment of
>> max(MAX_ORDER - 1, pageblock_order), because when MIGRATE_CMA was introduced,
>> __free_one_page() does not prevent merging two different pageblocks, when
>> MAX_ORDER - 1 > pageblock_order. But current __free_one_page() implementation
>> does prevent that.
>
> But it does prevent that only for isolated pageblock, not CMA, and yout
> patchset doesn't seem to expand that to CMA? Or am I missing something.

Yeah, you are right. Originally, I thought preventing merging isolated pageblock
with other types of pageblocks is sufficient, since MIGRATE_CMA is always
converted from MIGRATE_ISOLATE. But that is not true. I will rework the code.
Thanks for pointing this out.

>
>
>> It should be OK to just align cma to pageblock_order.
>> alloc_contig_range() relies on MIGRATE_CMA to get free pages, so it can use
>> pageblock_order as alignment too.
>>
>> In terms of virtio_mem, if I understand correctly, it relies on
>> alloc_contig_range() to obtain contiguous free pages and offlines them to reduce
>> guest memory size. As the result of alloc_contig_range() alignment change,
>> virtio_mem should be able to just align PFNs to pageblock_order.
>>
>> Thanks.
>>
>>
>> [1] https://lore.kernel.org/linux-mm/28b57903-fae6-47ac-7e1b-a1dd41421349@redhat.com/
>>
>> Zi Yan (3):
>>   mm: cma: alloc_contig_range: use pageblock_order as the single
>>     alignment.
>>   drivers: virtio_mem: use pageblock size as the minimum virtio_mem
>>     size.
>>   arch: powerpc: adjust fadump alignment to be pageblock aligned.
>>
>>  arch/powerpc/include/asm/fadump-internal.h |  4 +---
>>  drivers/virtio/virtio_mem.c                |  6 ++----
>>  include/linux/mmzone.h                     |  5 +----
>>  kernel/dma/contiguous.c                    |  2 +-
>>  mm/cma.c                                   |  6 ++----
>>  mm/page_alloc.c                            | 12 +++++-------
>>  6 files changed, 12 insertions(+), 23 deletions(-)
>>

--
Best Regards,
Yan, Zi
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