On 2021-12-09 8:31 p.m., Alistair Popple wrote:
On Friday, 10 December 2021 3:54:31 AM AEDT Sierra Guiza, Alejandro (Alex) wrote:
On 12/9/2021 10:29 AM, Felix Kuehling wrote:
Am 2021-12-09 um 5:53 a.m. schrieb Alistair Popple:
On Thursday, 9 December 2021 5:55:26 AM AEDT Sierra Guiza, Alejandro (Alex) wrote:
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.
I don't think that's true. The check in pfn_swap_entry_to_page() is only for
migration entries:
BUG_ON(is_migration_entry(entry) && !PageLocked(p));
As this is coherent memory though why do we have to call into a device driver
to do the migration? Couldn't this all be done in the kernel?
I think you're right. I hadn't thought of that mainly because I'm even
less familiar with the non-device migration code. Alex, can you give
that a try? As long as the driver still gets a page-free callback when
the device page is freed, it should work.
Yes, you should still get the page-free callback when the migration code drops
the last page reference.
ACK.Will do
There is currently not really any support for migrating device pages based on
pfn. What I think is needed is something like migrate_pages(), but that API
won't work for a couple of reasons - main one being that it relies on pages
being LRU pages.
I've been working on a series to implement an equivalent of migrate_pages() for
device-private (and by extension device-coherent) pages. It might also be useful
here so I will try and get it posted as an RFC next week.
If we want to make progress on this patch series in the shorter term, we
could just fail get_user_pages with FOLL_LONGTERM for DEVICE_COHERENT
pages. Then add the migration support when your patch series is ready.
Regards,
Felix
- Alistair
Alex Sierra
Regards,
Felix
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
