Re: [PATCH v2 03/11] mm/gup: migrate PIN_LONGTERM dev coherent pages to system

Alistair Popple
2021-12-08T11:32:15Z

On Tuesday, 7 December 2021 5:52:43 AM AEDT Alex Sierra wrote:
> Avoid long term pinning for Coherent device type pages. This could
> interfere with their own device memory manager.
> If caller tries to get user device coherent pages with PIN_LONGTERM flag
> set, those pages will be migrated back to system memory.
>
> Signed-off-by: Alex Sierra <alex.sierra@amd.com>
> ---
> mm/gup.c | 32 ++++++++++++++++++++++++++++++--
> 1 file changed, 30 insertions(+), 2 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 886d6148d3d0..1572eacf07f4 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1689,17 +1689,37 @@ struct page *get_dump_page(unsigned long addr)
> #endif /* CONFIG_ELF_CORE */
>
> #ifdef CONFIG_MIGRATION
> +static int migrate_device_page(unsigned long address,
> + struct page *page)
> +{
> + struct vm_area_struct *vma = find_vma(current->mm, address);
> + struct vm_fault vmf = {
> + .vma = vma,
> + .address = address & PAGE_MASK,
> + .flags = FAULT_FLAG_USER,
> + .pgoff = linear_page_index(vma, address),
> + .gfp_mask = GFP_KERNEL,
> + .page = page,
> + };
> + if (page->pgmap && page->pgmap->ops->migrate_to_ram)
> + return page->pgmap->ops->migrate_to_ram(&vmf);

How does this synchronise against pgmap being released? As I understand things
at this point we're not holding a reference on either the page or pgmap, so
the page and therefore the pgmap may have been freed.

I think a similar problem exists for device private fault handling as well and
it has been on my list of things to fix for a while. I think the solution is to
call try_get_page(), except it doesn't work with device pages due to the whole
refcount thing. That issue is blocking a fair bit of work now so I've started
looking into it.

> +
> + return -EBUSY;
> +}
> +
> /*
> * Check whether all pages are pinnable, if so return number of pages. If some
> * pages are not pinnable, migrate them, and unpin all pages. Return zero if
> * pages were migrated, or if some pages were not successfully isolated.
> * Return negative error if migration fails.
> */
> -static long check_and_migrate_movable_pages(unsigned long nr_pages,
> +static long check_and_migrate_movable_pages(unsigned long start,
> + unsigned long nr_pages,
> struct page **pages,
> unsigned int gup_flags)
> {
> unsigned long i;
> + unsigned long page_index;
> unsigned long isolation_error_count = 0;
> bool drain_allow = true;
> LIST_HEAD(movable_page_list);
> @@ -1720,6 +1740,10 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
> * If we get a movable page, since we are going to be pinning
> * these entries, try to move them out if possible.
> */
> + if (is_device_page(head)) {
> + page_index = i;
> + goto unpin_pages;
> + }
> if (!is_pinnable_page(head)) {
> if (PageHuge(head)) {
> if (!isolate_huge_page(head, &movable_page_list))
> @@ -1750,12 +1774,16 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
> if (list_empty(&movable_page_list) && !isolation_error_count)
> return nr_pages;
>
> +unpin_pages:
> if (gup_flags & FOLL_PIN) {
> unpin_user_pages(pages, nr_pages);
> } else {
> for (i = 0; i < nr_pages; i++)
> put_page(pages[i]);
> }
> + if (is_device_page(head))
> + return migrate_device_page(start + page_index * PAGE_SIZE, head);

This isn't very optimal - if a range contains more than one device page (which
seems likely) we will have to go around the whole gup/check_and_migrate loop
once for each device page which seems unnecessary. You should be able to either
build a list or migrate them as you go through the loop. I'm also currently
looking into how to extend migrate_pages() to support device pages which might
be useful here too.

> +
> if (!list_empty(&movable_page_list)) {
> ret = migrate_pages(&movable_page_list, alloc_migration_target,
> NULL, (unsigned long)&mtc, MIGRATE_SYNC,
> @@ -1798,7 +1826,7 @@ static long __gup_longterm_locked(struct mm_struct *mm,
> NULL, gup_flags);
> if (rc <= 0)
> break;
> - rc = check_and_migrate_movable_pages(rc, pages, gup_flags);
> + rc = check_and_migrate_movable_pages(start, rc, pages, gup_flags);
> } while (!rc);
> memalloc_pin_restore(flags);
>
>