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

[Daniel Jordan]

Jason Gunthorpe <jgg@xxxxxxxx> writes:
> On Fri, Dec 04, 2020 at 03:05:46PM -0500, Daniel Jordan wrote:
>> 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.
>
> iommu restrictions are not related to with gup. vfio needs to get the
> page list from the page tables as efficiently as possible, then you
> break it up into what you want to feed into the IOMMU how the iommu
> wants.
>
> vfio must maintain a page list to call unpin_user_pages() anyhow, so

It does in some cases but not others, namely the expensive
VFIO_IOMMU_MAP_DMA/UNMAP_DMA path where the iommu page tables are used
to find the pfns when unpinning.

I don't see why vfio couldn't do as you say, though, and the worst case
memory overhead of using scatterlist to remember the pfns of a 300g VM
backed by huge but physically discontiguous pages is only a few meg, not
bad at all.

> it makes alot of sense to assemble the page list up front, then do the
> iommu, instead of trying to do both things page at a time.
>
> It would be smart to rebuild vfio to use scatter lists to store the
> page list and then break the sgl into pages for iommu
> configuration. SGLs will consume alot less memory for the usual case
> of THPs backing the VFIO registrations.
>
> ib_umem_get() has some example of how to code this, I've been thinking
> we could make this some common API, and it could be further optimized.

Agreed, great suggestions, both above and in the rest of your response.

>> 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.
>
> Please don't just hack up vfio like this.

Yeah, you've cured me of that idea.  I'll see where I get experimenting
with some of this stuff.