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

[Joao Martins <joao.m.martins@oracle.com>]

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/