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

[Zi Yan <ziy@nvidia.com>]

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On 10 Dec 2021, at 13:36, David Hildenbrand wrote:

> 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.
>

OK. Make sense. I will add it in the next version.


--
Best Regards,
Yan, Zi

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