Re: [PATCH v1 07/11] mm/sparse-vmemmap: populate compound pagemaps

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jun 16, 2021 at 8:06 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 5/13/21 7:45 PM, Joao Martins wrote:
> > On 5/10/21 8:19 PM, Dan Williams wrote:
> >> On Thu, May 6, 2021 at 4:02 AM Joao Martins <joao.m.martins@oracle.com> wrote:
> >> [..]
> >>>>> +static pte_t * __meminit vmemmap_lookup_address(unsigned long addr)
> >>>>
> >>>> I think this can be replaced with a call to follow_pte(&init_mm...).
> >>>
> >>> Ah, of course! That would shorten things up too.
> >>
> >> Now that I look closely, I notice that function disclaims being
> >> suitable as a general purpose pte lookup utility.
> >> If it works for you,
> >> great, but if not I think it's past time to create such a helper. I
> >> know Ira is looking for one, and I contributed to the proliferation
> >> when I added dev_pagemap_mapping_shift().
> >>
> > There's also x86 equivalents, like lookup_address() and lookup_address_in_pgd().
> > These two don't differ that much from vmemmap_lookup_address().
> >
> > I can move this to an internal place e.g. mm/internal.h
> >
> > The patch after this one, changes vmemmap_lookup_address() to stop at the PMD (to reuse
> > that across the next sections).
> >
> Thinking again on this particular comment, but more on the actual need for the lookup
> function. It is very specific to 1G geometry case being spread over multiple sections.
>
> Given section population *needs* to succeed for the followup sections to continue to be
> populated, perhaps we simplify this a whole lot e.g. below comparing this patch before and
> after.
>
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 3d671e3e804d..c06796fcc77d 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -554,37 +554,6 @@ static inline int __meminit vmemmap_populate_page(unsigned long addr,
> int node,
>         return vmemmap_populate_address(addr, node, NULL, NULL, page);
>  }
>
> -static pte_t * __meminit vmemmap_lookup_address(unsigned long addr)
> -{
> -       pgd_t *pgd;
> -       p4d_t *p4d;
> -       pud_t *pud;
> -       pmd_t *pmd;
> -       pte_t *pte;
> -
> -       pgd = pgd_offset_k(addr);
> -       if (pgd_none(*pgd))
> -               return NULL;
> -
> -       p4d = p4d_offset(pgd, addr);
> -       if (p4d_none(*p4d))
> -               return NULL;
> -
> -       pud = pud_offset(p4d, addr);
> -       if (pud_none(*pud))
> -               return NULL;
> -
> -       pmd = pmd_offset(pud, addr);
> -       if (pmd_none(*pmd))
> -               return NULL;
> -
> -       pte = pte_offset_kernel(pmd, addr);
> -       if (pte_none(*pte))
> -               return NULL;
> -
> -       return pte;
> -}
> -
>  static int __meminit vmemmap_populate_compound_pages(unsigned long start_pfn,
>                                                      unsigned long start,
>                                                      unsigned long end, int node,
> @@ -605,8 +574,10 @@ static int __meminit vmemmap_populate_compound_pages(unsigned long
> start_pfn,
>         offset = PFN_PHYS(start_pfn) - pgmap->ranges[pgmap->nr_range].start;
>         if (!IS_ALIGNED(offset, pgmap_geometry(pgmap)) &&
>             pgmap_geometry(pgmap) > SUBSECTION_SIZE) {
> -               pte_t *ptep = vmemmap_lookup_address(start - PAGE_SIZE);
> +               pte_t *ptep;
>
> +               addr = start - PAGE_SIZE;
> +               ptep = pte_offset_kernel(pmd_off_k(addr), addr);

It deserves a comment about the guarantee, but looks good.

>                 if (!ptep)
>                         return -ENOMEM;
>
> The 'if (!ptep)' cannot happen in pratice AFAIU so could remove that as well.
>
> Thoughts?