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

[Daniel Jordan]

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.