linux-ext4 - Re: [PATCH v2 03/11] mm/gup: migrate PIN_LONGTERM dev coherent pages to system

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Date:   Thu, 9 Dec 2021 11:29:55 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>
Cc:     amd-gfx@...ts.freedesktop.org, willy@...radead.org,
        jglisse@...hat.com, dri-devel@...ts.freedesktop.org,
        jgg@...dia.com, hch@....de
Subject: Re: [PATCH v2 03/11] mm/gup: migrate PIN_LONGTERM dev coherent pages
 to system

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@....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.

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
>>>
>>>
>
>