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A compound devmap is a dev_pagemap with @vmemmap_shift > 0 and it means that pages are mapped at a given huge page alignment and utilize uses compound pages as opposed to order-0 pages. Take advantage of the fact that most tail pages look the same (except the first two) to minimize struct page overhead. Allocate a separate page for the vmemmap area which contains the head page and separate for the next 64 pages. The rest of the subsections then reuse this tail vmemmap page to initialize the rest of the tail pages. Sections are arch-dependent (e.g. on x86 it's 64M, 128M or 512M) and when initializing compound devmap with big enough @vmemmap_shift (e.g. 1G PUD) it may cross multiple sections. The vmemmap code needs to consult @pgmap so that multiple sections that all map the same tail data can refer back to the first copy of that data for a given gigantic page. On compound devmaps with 2M align, this mechanism lets 6 pages be saved out of the 8 necessary PFNs necessary to set the subsection's 512 struct pages being mapped. On a 1G compound devmap it saves 4094 pages. Altmap isn't supported yet, given various restrictions in altmap pfn allocator, thus fallback to the already in use vmemmap_populate(). It is worth noting that altmap for devmap mappings was there to relieve the pressure of inordinate amounts of memmap space to map terabytes of pmem. With compound pages the motivation for altmaps for pmem gets reduced. Link: https://lkml.kernel.org/r/20220420155310.9712-5-joao.m.martins@oracle.com Signed-off-by: Joao Martins <joao.m.martins@oracle.com> Reviewed-by: Muchun Song <songmuchun@bytedance.com> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Jane Chu <jane.chu@oracle.com> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Matthew Wilcox <willy@infradead.org> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Vishal Verma <vishal.l.verma@intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
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| .. | ||
| damon | ||
| .gitignore | ||
| active_mm.rst | ||
| arch_pgtable_helpers.rst | ||
| balance.rst | ||
| free_page_reporting.rst | ||
| frontswap.rst | ||
| highmem.rst | ||
| hmm.rst | ||
| hugetlbfs_reserv.rst | ||
| hwpoison.rst | ||
| index.rst | ||
| ksm.rst | ||
| memory-model.rst | ||
| mmu_notifier.rst | ||
| numa.rst | ||
| overcommit-accounting.rst | ||
| page_frags.rst | ||
| page_migration.rst | ||
| page_owner.rst | ||
| page_table_check.rst | ||
| remap_file_pages.rst | ||
| slub.rst | ||
| split_page_table_lock.rst | ||
| transhuge.rst | ||
| unevictable-lru.rst | ||
| vmalloced-kernel-stacks.rst | ||
| vmemmap_dedup.rst | ||
| z3fold.rst | ||
| zsmalloc.rst | ||