Re: [PATCH] drm/gem-shmem: Install writable PTEs for write faults

[Boris Brezillon <boris.brezillon@collabora.com>]

Hello Konstantin,

On Tue,  4 Aug 2026 17:04:03 +0300
Konstantin Fastov <kfastov@gmail.com> wrote:

> Since the introduction of dirty tracking, drm_gem_shmem_vm_ops has a
> .pfn_mkwrite handler.  Its presence makes vma_wants_writenotify() true,
> so vma_set_page_prot() removes the write bit from vm_page_prot of
> shared mappings, and the vmf_insert_pfn() call in the fault handler now
> installs read-only PTEs even when serving a write fault.
> 
> For regular CPU accesses this is transparent: the retried access faults
> again on the present read-only PTE, goes through wp_pfn_shared() into
> .pfn_mkwrite() and the PTE is upgraded to writable.  But consumers that
> resolve faults through fixup_user_fault() + follow_pfnmap_start()
> perform no such retry.  In particular KVM's hva_to_pfn_remapped(),
> after "successfully" handling a write fault, finds a present read-only
> PTE, treats it as KVM_PFN_ERR_RO_FAULT and fails the vcpu run with
> EFAULT.  Observed as AsahiLinux/linux#560: muvm/libkrun microVMs
> mapping virtio-gpu blob resources die with EFAULT on first GPU access.
> The traced failing sequence:
> 
>   follow_pfnmap_start()        -> -EINVAL   (no PTE yet)
>   fixup_user_fault(WRITE)
>     drm_gem_shmem_fault()
>       vmf_insert_pfn()         -> NOPAGE    (read-only PTE installed)
>   fixup_user_fault()           -> 0         (fault "handled")
>   follow_pfnmap_start()        -> 0, !writable
>   hva_to_pfn()                 -> KVM_PFN_ERR_RO_FAULT
> 
> Fix it the same way commit cb2a2a5b37ad ("drm/shmem_helper: Make sure
> PMD entries get the writeable upgrade") did for the PMD path: when the
> fault is a write fault, install a writable entry directly and record
> the write for dirty tracking, instead of relying on a refault that not
> every fault-resolution path performs.
> 
> To do that at PTE level, add vmf_insert_pfn_mkwrite(), the VM_PFNMAP
> counterpart of vmf_insert_mixed_mkwrite(): insert_pfn() already
> implements the mkwrite semantics, there was just no wrapper exposing it
> for pfn inserts with the default pgprot.

The same issue is being discussed here [1].

Regards,

Boris

[1]https://lore.kernel.org/dri-devel/20260804120529.1730187-1-pbonzini@redhat.com/