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

[Alistair Popple <apopple@nvidia.com>]

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@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.
> >> 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?

> 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
> >
> >
>