Re: [PATCH 3/3] mm/devmap: Remove pgmap accounting in the get_user_pages_fast() path

[Dan Williams]

On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
>
> On 3/18/21 4:08 AM, Dan Williams wrote:
> > Now that device-dax and filesystem-dax are guaranteed to unmap all user
> > mappings of devmap / DAX pages before tearing down the 'struct page'
> > array, get_user_pages_fast() can rely on its traditional synchronization
> > method "validate_pte(); get_page(); revalidate_pte()" to catch races with
> > device shutdown. Specifically the unmap guarantee ensures that gup-fast
> > either succeeds in taking a page reference (lock-less), or it detects a
> > need to fall back to the slow path where the device presence can be
> > revalidated with locks held.
>
> [...]
>
> > @@ -2087,21 +2078,26 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >  #endif /* CONFIG_ARCH_HAS_PTE_SPECIAL */
> >
> >  #if defined(CONFIG_ARCH_HAS_PTE_DEVMAP) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > +
> >  static int __gup_device_huge(unsigned long pfn, unsigned long addr,
> >                            unsigned long end, unsigned int flags,
> >                            struct page **pages, int *nr)
> >  {
> >       int nr_start = *nr;
> > -     struct dev_pagemap *pgmap = NULL;
> >
> >       do {
> > -             struct page *page = pfn_to_page(pfn);
> > +             struct page *page;
> > +
> > +             /*
> > +              * Typically pfn_to_page() on a devmap pfn is not safe
> > +              * without holding a live reference on the hosting
> > +              * pgmap. In the gup-fast path it is safe because any
> > +              * races will be resolved by either gup-fast taking a
> > +              * reference or the shutdown path unmapping the pte to
> > +              * trigger gup-fast to fall back to the slow path.
> > +              */
> > +             page = pfn_to_page(pfn);
> >
> > -             pgmap = get_dev_pagemap(pfn, pgmap);
> > -             if (unlikely(!pgmap)) {
> > -                     undo_dev_pagemap(nr, nr_start, flags, pages);
> > -                     return 0;
> > -             }
> >               SetPageReferenced(page);
> >               pages[*nr] = page;
> >               if (unlikely(!try_grab_page(page, flags))) {
>
> So for allowing FOLL_LONGTERM[0] would it be OK if we used page->pgmap after
> try_grab_page() for checking pgmap type to see if we are in a device-dax
> longterm pin?

So, there is an effort to add a new pte bit p{m,u}d_special to disable
gup-fast for huge pages [1]. I'd like to investigate whether we could
use devmap + special as an encoding for "no longterm" and never
consult the pgmap in the gup-fast path.

[1]: https://lore.kernel.org/linux-mm/a1fa7fa2-914b-366d-9902-e5b784e8428c@xxxxxxxxxxxx/