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

[Alistair Popple <apopple@nvidia.com>]

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@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?
> > 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.

 - 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
> >>>>
> >>>>
> >>