Re: [PATCH v3 5/8] mm: Device exclusive memory access

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Feb 26, 2021 at 06:18:29PM +1100, Alistair Popple wrote:

> +/**
> + * make_device_exclusive_range() - Mark a range for exclusive use by a device
> + * @mm: mm_struct of assoicated target process
> + * @start: start of the region to mark for exclusive device access
> + * @end: end address of region
> + * @pages: returns the pages which were successfully mark for exclusive acces
> + *
> + * Returns: number of pages successfully marked for exclusive access
> + *
> + * This function finds the ptes mapping page(s) to the given address range and
> + * replaces them with special swap entries preventing userspace CPU access. On
> + * fault these entries are replaced with the original mapping after calling MMU
> + * notifiers.
> + */
> +int make_device_exclusive_range(struct mm_struct *mm, unsigned long start,
> +				unsigned long end, struct page **pages)
> +{
> +	long npages = (end - start) >> PAGE_SHIFT;
> +	long i;
> +
> +	npages = get_user_pages_remote(mm, start, npages,
> +				       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
> +				       pages, NULL, NULL);
> +	for (i = 0; i < npages; i++) {
> +		if (!trylock_page(pages[i])) {
> +			put_page(pages[i]);
> +			pages[i] = NULL;
> +			continue;
> +		}
> +
> +		if (!try_to_protect(pages[i])) {

Isn't this racy? get_user_pages returns the ptes at an instant in
time, they could have already been changed to something else?

I would think you'd want to switch to the swap entry atomically under
th PTLs?

Jason