On 11.02.22 00:41, Alistair Popple wrote:
> On Thursday, 10 February 2022 10:47:35 PM AEDT David Hildenbrand wrote:
>> On 10.02.22 12:39, Alistair Popple wrote:
>>> On Thursday, 10 February 2022 9:53:38 PM AEDT David Hildenbrand wrote:
>>>> On 07.02.22 05:26, Alistair Popple wrote:
>>>>> Currently any attempts to pin a device coherent page will fail. This is
>>>>> because device coherent pages need to be managed by a device driver, and
>>>>> pinning them would prevent a driver from migrating them off the device.
>>>>>
>>>>> However this is no reason to fail pinning of these pages. These are
>>>>> coherent and accessible from the CPU so can be migrated just like
>>>>> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
>>>>> them first try migrating them out of ZONE_DEVICE.
>>>>>
>>>>> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
>>>>> Acked-by: Felix Kuehling <Felix.Kuehling@xxxxxxx>
>>>>> ---
>>>>>
>>>>> Changes for v2:
>>>>>
>>>>> - Added Felix's Acked-by
>>>>> - Fixed missing check for dpage == NULL
>>>>>
>>>>> mm/gup.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++------
>>>>> 1 file changed, 95 insertions(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>> index 56d9577..5e826db 100644
>>>>> --- a/mm/gup.c
>>>>> +++ b/mm/gup.c
>>>>> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
>>>>>
>>>>> #ifdef CONFIG_MIGRATION
>>>>> /*
>>>>> + * Migrates a device coherent page back to normal memory. Caller should have a
>>>>> + * reference on page which will be copied to the new page if migration is
>>>>> + * successful or dropped on failure.
>>>>> + */
>>>>> +static struct page *migrate_device_page(struct page *page,
>>>>> + unsigned int gup_flags)
>>>>> +{
>>>>> + struct page *dpage;
>>>>> + struct migrate_vma args;
>>>>> + unsigned long src_pfn, dst_pfn = 0;
>>>>> +
>>>>> + lock_page(page);
>>>>> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
>>>>> + args.src = &src_pfn;
>>>>> + args.dst = &dst_pfn;
>>>>> + args.cpages = 1;
>>>>> + args.npages = 1;
>>>>> + args.vma = NULL;
>>>>> + migrate_vma_setup(&args);
>>>>> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
>>>>> + return NULL;
>>>>> +
>>>>> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
>>>>> +
>>>>> + /*
>>>>> + * get/pin the new page now so we don't have to retry gup after
>>>>> + * migrating. We already have a reference so this should never fail.
>>>>> + */
>>>>> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
>>>>> + __free_pages(dpage, 0);
>>>>> + dpage = NULL;
>>>>> + }
>>>>> +
>>>>> + if (dpage) {
>>>>> + lock_page(dpage);
>>>>> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
>>>>> + }
>>>>> +
>>>>> + migrate_vma_pages(&args);
>>>>> + if (src_pfn & MIGRATE_PFN_MIGRATE)
>>>>> + copy_highpage(dpage, page);
>>>>> + migrate_vma_finalize(&args);
>>>>> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
>>>>> + if (gup_flags & FOLL_PIN)
>>>>> + unpin_user_page(dpage);
>>>>> + else
>>>>> + put_page(dpage);
>>>>> + dpage = NULL;
>>>>> + }
>>>>> +
>>>>> + return dpage;
>>>>> +}
>>>>> +
>>>>> +/*
>>>>> * Check whether all pages are pinnable, if so return number of pages. If some
>>>>> * pages are not pinnable, migrate them, and unpin all pages. Return zero if
>>>>> * pages were migrated, or if some pages were not successfully isolated.
>>>>> @@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
>>>>> continue;
>>>>> prev_head = head;
>>>>> /*
>>>>> - * If we get a movable page, since we are going to be pinning
>>>>> - * these entries, try to move them out if possible.
>>>>> + * Device coherent pages are managed by a driver and should not
>>>>> + * be pinned indefinitely as it prevents the driver moving the
>>>>> + * page. So when trying to pin with FOLL_LONGTERM instead try
>>>>> + * migrating page out of device memory.
>>>>> */
>>>>> if (is_dev_private_or_coherent_page(head)) {
>>>>> + /*
>>>>> + * device private pages will get faulted in during gup
>>>>> + * so it shouldn't be possible to see one here.
>>>>> + */
>>>>> WARN_ON_ONCE(is_device_private_page(head));
>>>>> - ret = -EFAULT;
>>>>> - goto unpin_pages;
>>>>> + WARN_ON_ONCE(PageCompound(head));
>>>>> +
>>>>> + /*
>>>>> + * migration will fail if the page is pinned, so convert
>>>>> + * the pin on the source page to a normal reference.
>>>>> + */
>>>>> + if (gup_flags & FOLL_PIN) {
>>>>> + get_page(head);
>>>>> + unpin_user_page(head);
>>>>> + }
>>>>> +
>>>>> + pages[i] = migrate_device_page(head, gup_flags);
>>>>
>>>> For ordinary migrate_pages(), we'll unpin all pages and return 0 so the
>>>> caller will retry pinning by walking the page tables again.
>>>>
>>>> Why can't we apply the same mechanism here? This "let's avoid another
>>>> walk" looks unnecessary complicated to me, but I might be wrong.
>>>
>>> There's no reason we couldn't. I figured we have the page in the right spot
>>> anyway so it was easy to do, and looking at this rebased on top of Christoph's
>>> ZONE_DEVICE refcount simplification I'm not sure it would be any simpler
>>> anyway.
>>>
>>> It would remove the call to try_grab_page(), but we'd still have to return an
>>> error on migration failures whilst also ensuring we putback any non-device
>>> pages that may have been isolated. I might have overlooked something though,
>>> so certainly happy for suggestions.
>>
>> Staring at the code, I was wondering if we could either
>>
>> * build a second list of device coherent pages to migrate and call a
>> migrate_device_pages() bulk function
>> * simply use movable_page_list() and teach migrate_pages() how to handle
>> them.
>
(sorry for the late reply)
> I did consider that approach. The problem is zone device pages are not LRU
> pages. In particular page->lru is not available to add the page to a list, and
> as an external API and internally migrate_pages() relies heavily on moving
> pages between lists.
I see, so I assume there is no way we could add them to a list? We could
use a temporary array we'd try allocating once we stumble over over such
a device page that needs migration.
The you'd teach is_pinnable_page() to reject
is_dev_private_or_coherent_page() and special case
is_dev_private_or_coherent_page() under the "if
(!is_pinnable_page(head))" check.
You'd grab an additional reference and add them to the temp array. The
you'd just proceed as normal towards the end of the function (reverting
the pin/ref from the input array) and would try to migrate all device
pages in the temp array just before migrate_pages() --
migrate_device_pages(), properly handling/indicating if either migration
path fails.
Instead of putback_movable_pages() on the list you'd had
unref_device_pages() and supply the array.
Just a thought to limit the impact and eventually make it a bit nicer to
read, avoiding modifications of the input array.
>
>> I'd really appreciate as little special casing as possible for the ever
>> growing list of new DEVICE types all over the place. E.g., just staring
>> at fork even before the new device coherent made my head spin.
>
> That's fair. We could pull the checks for device pages out into a self
> contained function (eg. check_and_migrate_device_pages()) called before
> check_and_migrate_movable_pages(). The down side of that is we'd always have an
> extra loop over all the pages just to scan for device pages, but perhaps that's
> not a concern?
I mean, they are movable ... just not "ordinarily" movable, so it smells
like this logic belongs into check_and_migrate_movable_pages() :)
--
Thanks,
David / dhildenb
