[PATCH v1 0/2] udmabuf: Add back support for mapping hugetlb pages

[Jason Gunthorpe]

On Fri, Jun 23, 2023 at 12:35:45PM -0400, Peter Xu wrote:

> It seems the previous concern on using gup was majorly fork(), if this is it:
> 
> https://patchwork.freedesktop.org/patch/210992/?series=39879&rev=2#comment_414213

Fork and GUP have been fixed since that comment anyhow there is no
longer a problem using GUP and fork together.

> Could it also be guarded by just making sure the pages are MAP_SHARED when
> creating the udmabuf, if fork() is a requirement of the feature?

Also a resaonable thing to do

> For instance, what if the userapp just punchs a hole in the shmem/hugetlbfs
> file after creating the udmabuf (I see that F_SEAL_SHRINK is required, but
> at least not F_SEAL_WRITE with current impl), and fault a new page into the
> page cache?

It becomes incoherent just like all other GUP users if userspace
explicitly manipulates the VMAs. It is no different to what happens
with VFIO, qemu should treat this memory the same as it does for VFIO
memory.

Jason