On 12/8/2021 11:30 AM, Felix Kuehling wrote:
Am 2021-12-08 um 11:58 a.m. schrieb Felix Kuehling:
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:
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@xxxxxxx>
---
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.
Never mind. We unpin the pages first. Alex, would the migration work if
we unpinned them afterwards? Also, the normal CPU page fault code path
seems to make sure the page is locked (check in pfn_swap_entry_to_page)
before calling migrate_to_ram.
No, you can not unpinned after migration. Due to the expected_count VS
page_count condition at migrate_page_move_mapping, during migrate_page call.
Regards,
Alex Sierra
Regards,
Felix
