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

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);