On 08.12.20 18:28, Joao Martins wrote: > Hey, > > This small series, attempts at minimizing 'struct page' overhead by > pursuing a similar approach as Muchun Song series "Free some vmemmap > pages of hugetlb page"[0] but applied to devmap/ZONE_DEVICE. > > [0] https://lore.kernel.org/linux-mm/20201130151838.11208-1-songmuchun@xxxxxxxxxxxxx/ > > The link above describes it quite nicely, but the idea is to reuse tail > page vmemmap areas, particular the area which only describes tail pages. > So a vmemmap page describes 64 struct pages, and the first page for a given > ZONE_DEVICE vmemmap would contain the head page and 63 tail pages. The second > vmemmap page would contain only tail pages, and that's what gets reused across > the rest of the subsection/section. The bigger the page size, the bigger the > savings (2M hpage -> save 6 vmemmap pages; 1G hpage -> save 4094 vmemmap pages). > > In terms of savings, per 1Tb of memory, the struct page cost would go down > with compound pagemap: > > * with 2M pages we lose 4G instead of 16G (0.39% instead of 1.5% of total memory) > * with 1G pages we lose 8MB instead of 16G (0.0007% instead of 1.5% of total memory) > That's the dream :) > Along the way I've extended it past 'struct page' overhead *trying* to address a > few performance issues we knew about for pmem, specifically on the > {pin,get}_user_pages* function family with device-dax vmas which are really > slow even of the fast variants. THP is great on -fast variants but all except > hugetlbfs perform rather poorly on non-fast gup. > > So to summarize what the series does: > > Patches 1-5: Much like Muchun series, we reuse tail page areas across a given > page size (namely @align was referred by remaining memremap/dax code) and > enabling of memremap to initialize the ZONE_DEVICE pages as compound pages or a > given @align order. The main difference though, is that contrary to the hugetlbfs > series, there's no vmemmap for the area, because we are onlining it. Yeah, I'd argue that this case is a lot easier to handle. When the buddy is involved, things get more complicated. -- Thanks, David / dhildenb
