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

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))) {