Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps

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

On Tue, Feb 23, 2021 at 8:30 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 2/20/21 1:18 AM, Dan Williams wrote:
> > On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@oracle.com> wrote:
> >> Patch 6 - 8: Optimize grabbing/release a page refcount changes given that we
> >> are working with compound pages i.e. we do 1 increment/decrement to the head
> >> page for a given set of N subpages compared as opposed to N individual writes.
> >> {get,pin}_user_pages_fast() for zone_device with compound pagemap consequently
> >> improves considerably, and unpin_user_pages() improves as well when passed a
> >> set of consecutive pages:
> >>
> >>                                            before          after
> >>     (get_user_pages_fast 1G;2M page size) ~75k  us -> ~3.2k ; ~5.2k us
> >>     (pin_user_pages_fast 1G;2M page size) ~125k us -> ~3.4k ; ~5.5k us
> >
> > Compelling!
> >
>
> BTW is there any reason why we don't support pin_user_pages_fast() with FOLL_LONGTERM for
> device-dax?
>

Good catch.

Must have been an oversight of the conversion. FOLL_LONGTERM collides
with filesystem operations, but not device-dax. In fact that's the
motivation for device-dax in the first instance, no need to coordinate
runtime physical address layout changes because the device is
statically allocated.

> Looking at the history, I understand that fsdax can't support it atm, but I am not sure
> that the same holds for device-dax. I have this small chunk (see below the scissors mark)
> which relaxes this for a pgmap of type MEMORY_DEVICE_GENERIC, albeit not sure if there is
> a fundamental issue for the other types that makes this an unwelcoming change.
>
>         Joao
>
> --------------------->8---------------------
>
> Subject: [PATCH] mm/gup: allow FOLL_LONGTERM pin-fast for
>  MEMORY_DEVICE_GENERIC
>
> The downside would be one extra lookup in dev_pagemap tree
> for other pgmap->types (P2P, FSDAX, PRIVATE). But just one
> per gup-fast() call.

I'd guess a dev_pagemap lookup is faster than a get_user_pages slow
path. It should be measurable that this change is at least as fast or
faster than falling back to the slow path, but it would be good to
measure.

Code changes look good to me.

>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  include/linux/mm.h |  5 +++++
>  mm/gup.c           | 24 +++++++++++++-----------
>  2 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 32f0c3986d4f..c89a049bbd7a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1171,6 +1171,11 @@ static inline bool is_pci_p2pdma_page(const struct page *page)
>                 page->pgmap->type == MEMORY_DEVICE_PCI_P2PDMA;
>  }
>
> +static inline bool devmap_longterm_available(const struct dev_pagemap *pgmap)
> +{

I'd call this devmap_can_longterm().

> +       return pgmap->type == MEMORY_DEVICE_GENERIC;
> +}
> +
>  /* 127: arbitrary random number, small enough to assemble well */
>  #define page_ref_zero_or_close_to_overflow(page) \
>         ((unsigned int) page_ref_count(page) + 127u <= 127u)
> diff --git a/mm/gup.c b/mm/gup.c
> index 222d1fdc5cfa..03e370d360e6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2092,14 +2092,18 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned
> long end,
>                         goto pte_unmap;
>
>                 if (pte_devmap(pte)) {
> -                       if (unlikely(flags & FOLL_LONGTERM))
> -                               goto pte_unmap;
> -
>                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
>                         if (unlikely(!pgmap)) {
>                                 undo_dev_pagemap(nr, nr_start, flags, pages);
>                                 goto pte_unmap;
>                         }
> +
> +                       if (unlikely(flags & FOLL_LONGTERM) &&
> +                           !devmap_longterm_available(pgmap)) {
> +                               undo_dev_pagemap(nr, nr_start, flags, pages);
> +                               goto pte_unmap;
> +                       }
> +
>                 } else if (pte_special(pte))
>                         goto pte_unmap;
>
> @@ -2195,6 +2199,10 @@ static int __gup_device_huge(unsigned long pfn, unsigned long addr,
>                         return 0;
>                 }
>
> +               if (unlikely(flags & FOLL_LONGTERM) &&
> +                   !devmap_longterm_available(pgmap))
> +                       return 0;
> +
> @@ -2356,12 +2364,9 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>         if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
>                 return 0;
>
> -       if (pmd_devmap(orig)) {
> -               if (unlikely(flags & FOLL_LONGTERM))
> -                       return 0;
> +       if (pmd_devmap(orig))
>                 return __gup_device_huge_pmd(orig, pmdp, addr, end, flags,
>                                              pages, nr);
> -       }
>
>         page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>         refs = record_subpages(page, addr, end, pages + *nr);
> @@ -2390,12 +2395,9 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>         if (!pud_access_permitted(orig, flags & FOLL_WRITE))
>                 return 0;
>
> -       if (pud_devmap(orig)) {
> -               if (unlikely(flags & FOLL_LONGTERM))
> -                       return 0;
> +       if (pud_devmap(orig))
>                 return __gup_device_huge_pud(orig, pudp, addr, end, flags,
>                                              pages, nr);
> -       }
>
>         page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>         refs = record_subpages(page, addr, end, pages + *nr);
> --
> 2.17.1
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