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

[Thorsten Leemhuis]

[CCing Paolo and Sergio]

On 8/4/26 16:57, Konstantin Fastov wrote:
> вт, 4 авг. 2026 г. в 17:29, Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>:
>> On Tue,  4 Aug 2026 17:04:03 +0300
>> Konstantin Fastov <kfastov@xxxxxxxxx> 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: [...]
>>
>> The same issue is being discussed here [1].
>> [...]
>> [1]https://lore.kernel.org/dri-devel/20260804120529.1730187-1-pbonzini@xxxxxxxxxx/
> 
> 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 name), so please consider my patch superseded by his series. [...]
FWIW, it seems that series takes a while to get polished and mainlined:
https://lore.kernel.org/all/CABgObfYgZ87%3DW%2B6E%3DN%3DWwTuyYkaoKU3hqd84JDTSAvT3_9dh6w@xxxxxxxxxxxxxx/

To quote: "It ballooned a bit and getting acks from everyone involved is
hard... Either I or Sergio will get back to it."

Is that something you/AsahiLinux/the world can live with?

Ciao, Thorsten