Am 2021-12-08 um 6:31 a.m. schrieb Alistair Popple:

On Tuesday, 7 December 2021 5:52:43 AM AEDT Alex Sierra wrote:

quoted

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 <redacted>
---
 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.

At least the page should have been pinned by the __get_user_pages_locked
call in __gup_longterm_locked. That refcount is dropped in
check_and_migrate_movable_pages when it returns 0 or an error.

quoted

+
+	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.

We have to do it this way because page->pgmap->ops->migrate_to_ram can
migrate multiple pages per "CPU page fault" to amortize the cost of
migration. The AMD driver typically migrates 2MB at a time. Calling
page->pgmap->ops->migrate_to_ram for each page would probably be even
less optimal.

Regards,
  Felix

quoted

+
 	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);