Re: + fix-zero-copy-i-o-on-__get_user_pages-allocated-pages.patch added to mm-hotfixes-unstable branch

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, May 08, 2025 at 05:40:15PM +0200, David Hildenbrand wrote:
> > I don't think there was a deliberate decision here, but there was no
> > conversion to remap_pfn_range(), the code (in DRM) was always there.
> > 
> > The regression occurred when netfslib started using GUP for I/O and
> > when filesystems switched to it we hit this case.
> 
> Okay, so GUP and DRM always worked that way. They are essentially
> incompatible at this point due to VM_PFNMAP.
> 
> So netfslib requesting something that is impossible is the problem .. or
> rather filesystems switching to that and not realizing the problem.
> 
> Hmmm

This patch definately doesn't look very good as is. 

We *certainly* should not be even trying to touch the struct page of a
VMA_PFNMAP *at all*. By definition that is forbidden.

It looks to me like vm_normal_page() already supports MIXEDMAP, so
probably the better hotfix is to have DRM use MIXEDMAP if it is
installing PFNs that it is willing to be used as struct page.

But who knows if DRM can do that on arches that don't have
PTE_SPECIAL..

Jason