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

Joao

[0] https://lore.kernel.org/linux-mm/6a18179e-65f7-367d-89a9-d5162f10fef0@oracle.com/

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