On Fri, Mar 4, 2022 at 5:33 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: > > 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. > > Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx> Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Thanks.
