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SubjectRe: [RFC PATCH v2 0/7] Use pageblock_order for cma and alloc_contig_range alignment.
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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(-)
>

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