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

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

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

quoted

On 11/15/21 20:37, Zi Yan wrote:

quoted

From: Zi Yan <ziy@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.

quoted

quoted

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/ (local)

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