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

Hello Boris,

вт, 4 авг. 2026 г. в 17:29, Boris Brezillon <[email protected]>:
>
> Hello Konstantin,
>
> On Tue,  4 Aug 2026 17:04:03 +0300
> Konstantin Fastov <[email protected]> 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/[email protected]/

Thanks, I missed that series, I searched dri-devel but not the kvm list.
Paolo's patches 1-2 are indeed the same fix (down to the same function
name), so please consider my patch superseded by his series.

I have a fast reproducer on Apple Silicon (Asahi, muvm microVM, dies in
under a second on stock 7.1.5) and an automated test rig, so I'll run
Paolo's full series on that hardware and follow up with Tested-by in his
thread.

Thanks,
Konstantin