On Thu, Mar 18, 2021 at 3:02 AM Joao Martins <joao.m.martins(a)oracle.com> 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?
Yes. I still need to answer the question of whether mapping
invalidation triggers longterm pin holders to relinquish their hold,
but that's a problem regardless of whether gup-fast is supported or
not.