Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[David Hildenbrand]

> +static bool folio_longterm_write_pin_allowed(struct folio *folio)
> +{
> +	struct address_space *mapping;
> +
> +	/*
> +	 * GUP-fast disables IRQs - this prevents IPIs from causing page tables
> +	 * to disappear from under us, as well as preventing RCU grace periods
> +	 * from making progress (i.e. implying rcu_read_lock()).
> +	 *
> +	 * This means we can rely on the folio remaining stable for all
> +	 * architectures, both those that set CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +	 * and those that do not.
> +	 *
> +	 * We get the added benefit that given inodes, and thus address_space,
> +	 * objects are RCU freed, we can rely on the mapping remaining stable
> +	 * here with no risk of a truncation or similar race.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * If no mapping can be found, this implies an anonymous or otherwise
> +	 * non-file backed folio so in this instance we permit the pin.
> +	 *
> +	 * shmem and hugetlb mappings do not require dirty-tracking so we
> +	 * explicitly whitelist these.
> +	 *
> +	 * Other non dirty-tracked folios will be picked up on the slow path.
> +	 */
> +	mapping = folio_mapping(folio);
> +	return !mapping || shmem_mapping(mapping) || folio_test_hugetlb(folio);
> +}

BTW, try_grab_folio() is also called from follow_hugetlb_page(), which 
is ordinary GUP and has interrupts enabled if I am not wrong.

--
Thanks,

David / dhildenb