Re: [PATCH v7 4/5] mm/sparse-vmemmap: improve memory savings for compound devmaps

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Mar 4, 2022 at 5:33 AM Joao Martins <joao.m.martins@oracle.com> 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@oracle.com>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.