Re: [RFC PATCH v2 0/7] Use pageblock_order for cma and alloc_contig_range alignment.

[Eric Ren <renzhengeek@gmail.com>]

Hi Zi Yan,

On 2021/12/10 07:04, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
>
> Hi all,
>
> This patchset tries to remove the MAX_ORDER - 1 alignment requirement for CMA
> and alloc_contig_range(). It prepares for my upcoming changes to make MAX_ORDER
> adjustable at boot time[1].
>
> The MAX_ORDER - 1 alignment requirement comes from that alloc_contig_range()
> isolates pageblocks to remove free memory from buddy allocator but isolating
> only a subset of pageblocks within a page spanning across multiple pageblocks
> causes free page accounting issues. Isolated page might not be put into the
> right free list, since the code assumes the migratetype of the first pageblock
> as the whole free page migratetype. This is based on the discussion at [2].
>
> To remove the requirement, this patchset:
> 1. still isolates pageblocks at MAX_ORDER - 1 granularity;
Then, unplug fails if either pageblock of the  MAX_ORDER - 1 page has 
unmovable page, right?

Thanks,
Eric
> 2. but saves the pageblock migratetypes outside the specified range of
>     alloc_contig_range() and restores them after all pages within the range
>     become free after __alloc_contig_migrate_range();
> 3. splits free pages spanning multiple pageblocks at the beginning and the end
>     of the range and puts the split pages to the right migratetype free lists
>     based on the pageblock migratetypes;
> 4. returns pages not in the range as it did before this patch.
>
> Isolation needs to happen at MAX_ORDER - 1 granularity, because otherwise
> 1) extra code is needed to detect pages (free, PageHuge, THP, or PageCompound)
> to make sure all pageblocks belonging to a single page are isolated together
> and later pageblocks outside the range need to have their migratetypes restored;
> or 2) extra logic will need to be added during page free time to split a free
> page with multi-migratetype pageblocks.
>
> Two optimizations might come later:
> 1. only check unmovable pages within the range instead of MAX_ORDER - 1 aligned
>     range during isolation to increase successful rate of alloc_contig_range().
> 2. make MIGRATE_ISOLATE a separate bit to avoid saving and restoring existing
>     migratetypes before and after isolation respectively.
>
> Feel free to give comments and suggestions. Thanks.
>
>
> [1] https://lore.kernel.org/linux-mm/20210805190253.2795604-1-zi.yan@sent.com/
> [2] https://lore.kernel.org/linux-mm/d19fb078-cb9b-f60f-e310-fdeea1b947d2@redhat.com/
>
>
> Zi Yan (7):
>    mm: page_alloc: avoid merging non-fallbackable pageblocks with others.
>    mm: compaction: handle non-lru compound pages properly in
>      isolate_migratepages_block().
>    mm: migrate: allocate the right size of non hugetlb or THP compound
>      pages.
>    mm: make alloc_contig_range work at pageblock granularity
>    mm: cma: 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                     |  11 +-
>   kernel/dma/contiguous.c                    |   2 +-
>   mm/cma.c                                   |   6 +-
>   mm/compaction.c                            |  10 +-
>   mm/migrate.c                               |   8 +-
>   mm/page_alloc.c                            | 203 +++++++++++++++++----
>   8 files changed, 196 insertions(+), 54 deletions(-)
>