On 10 Dec 2021, at 13:36, David Hildenbrand wrote: > On 10.12.21 00:04, Zi Yan wrote: >> From: Zi Yan <ziy@xxxxxxxxxx> >> >> 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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