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

[Jason Gunthorpe]

On Mon, Dec 07, 2020 at 09:48:48PM -0500, Daniel Jordan wrote:
> 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.

Oh, I see.. Well, that is still possible, but vfio really needs to
batch operations, eg call pin_user_pages() with some larger buffer and
store those into the iommu and then reverse this to build up
contiguous runs of pages to unpin

> 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.

Yes, but 0 is still better.. I would start by focusing on batching
pin_user_pages.

Jason