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

[Zi Yan]

On 19 Nov 2021, at 10:15, Zi Yan wrote:

> 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.
>

I find that two pageblocks with different migratetypes, like MIGRATE_RECLAIMABLE
and MIGRATE_MOVABLE can be merged into a single free page after I checked
__free_one_page() in detail and printed pageblock information during buddy page
merging. I am not sure what consequence it will cause. Do you have any idea?

I will fix it in the next version of this patchset.

>>
>>
>>> 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


--
Best Regards,
Yan, Zi
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