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

[David Hildenbrand <david@redhat.com>]

On 10.12.21 00:04, Zi Yan wrote:
> From: Zi Yan <ziy@nvidia.com>
> 
> Hi all,

Hi,

thanks for working on that!

> 
> 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;
> 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().

The issue with virtio-mem is that we'll need that as soon as we change
the granularity to pageblocks, because otherwise, you can heavily
degrade unplug reliably in sane setups:

Previous:
* Try unplug free 4M range (2 pageblocks): succeeds

Now:
* Try unplug 2M range (first pageblock): succeeds.
* Try unplug next 2M range (second pageblock): fails because first
contains unmovable allcoations.

-- 
Thanks,

David / dhildenb