Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

[Daniel Jordan]

Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> writes:

> On Fri, Dec 4, 2020 at 3:06 PM Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> wrote:
>>
>> Jason Gunthorpe <jgg@xxxxxxxx> writes:
>>
>> > On Wed, Dec 02, 2020 at 08:34:32PM -0500, Pavel Tatashin wrote:
>> >> What I meant is the users of the interface do it incrementally not in
>> >> large chunks. For example:
>> >>
>> >> vfio_pin_pages_remote
>> >>    vaddr_get_pfn
>> >>         ret = pin_user_pages_remote(mm, vaddr, 1, flags |
>> >> FOLL_LONGTERM, page, NULL, NULL);
>> >> 1 -> pin only one pages at a time
>> >
>> > I don't know why vfio does this, it is why it so ridiculously slow at
>> > least.
>>
>> Well Alex can correct me, but I went digging and a comment from the
>> first type1 vfio commit says the iommu API didn't promise to unmap
>> subpages of previous mappings, so doing page at a time gave flexibility
>> at the cost of inefficiency.
>>
>> Then 166fd7d94afd allowed the iommu to use larger pages in vfio, but
>> vfio kept pinning pages at a time.  I couldn't find an explanation for
>> why that stayed the same.
>>
>> Yesterday I tried optimizing vfio to skip gup calls for tail pages after
>> Matthew pointed out this same issue to me by coincidence last week.
>> Currently debugging, but if there's a fundamental reason this won't work
>> on the vfio side, it'd be nice to know.
>
> Hi Daniel,
>
> I do not think there are any fundamental reasons why it won't work. I
> have also thinking increasing VFIO chunking for a different reason:
>
> If a client touches pages before doing a VFIO DMA map, those pages
> might be huge, and pinning a small page at a time and migrating a
> small page at a time can break-up the huge pages. So, it is not only
> inefficient to pin, but it can also inadvertently slow down the
> runtime.

Hi Pasha,

I see, and I'm curious, do you experience this case where a user has
touched the pages before doing a VFIO DMA map, and if so where?

The usual situation on my side is that the pages are faulted in during
pinning.

Daniel