Re: [PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 15, 2026, Pankaj Gupta wrote:
> > > Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
> > > encrypted memory regions") added FOLL_LONGTERM to
> > > sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
> > > MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
> > > FOLL_WRITE on the pin.
> > > 
> > > Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
> > > guest memory, such as virtio-pmem host memory-backend-file mappings
> > > (MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
> > > mappings since:
> > > 
> > > commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
> > > commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").
> > > 
> > > Region registration only requires long-term pin to prevent page migration and
> > > does not write through this GUP pin.
> > > 
> > > Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.
> > Worth mentioning here something like
> > 
> > "In the past, FOLL_WRITE was required to trigger CoW unsharing, making sure that
> > we don't end up replacing the page in the page tables during a later write fault
> > after already having pinned a (shared) page in MAP_PRIVATE mappings.
> > FOLL_LONGTERM does that nowadays (see gup_must_unshare()) even without FOLL_WRITE.

Heh, this was going to be my exact question about why it was safe to drop FOLL_WRITE :-)

> > Given that SEV only pins RAM for XYZ and doesn't actually write to the pinned
> > pages, we can just drop the FOLL_WRITE"
> > 
> > Fill out XYZ :)
> 
> Sure :)
> 
> I will update the commit message in v3.

No need for a v3, I'll add a blurb when applying.