Re: [PATCH v6 4/5] mm/sparse-vmemmap: improve memory savings for compound devmaps

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Feb 24, 2022 at 3:48 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>
[...]
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5f549cf6a4e8..b0798b9c6a6a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3118,7 +3118,7 @@ p4d_t *vmemmap_p4d_populate(pgd_t *pgd, unsigned long addr, int node);
>  pud_t *vmemmap_pud_populate(p4d_t *p4d, unsigned long addr, int node);
>  pmd_t *vmemmap_pmd_populate(pud_t *pud, unsigned long addr, int node);
>  pte_t *vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
> -                           struct vmem_altmap *altmap);
> +                           struct vmem_altmap *altmap, struct page *block);

Have forgotten to update @block to @reuse here.

[...]
> +
> +static int __meminit vmemmap_populate_range(unsigned long start,
> +                                           unsigned long end,
> +                                           int node, struct page *page)

All of the users are passing a valid parameter of @page. This function
will populate the vmemmap with the @page and without memory
allocations. So the @node parameter seems to be unnecessary.

If you want to make this function more generic like
vmemmap_populate_address() to handle memory allocations
(the case of @page == NULL). I think vmemmap_populate_range()
should add another parameter of `struct vmem_altmap *altmap`.
Otherwise, is it better to remove @node and rename @page to @reuse?

Thanks.