Re: [PATCH v2 2/6] drm/shmem_helper: use vmf_insert_pfn_mkwrite()

On Tue, Aug 4, 2026 at 4:19 PM Boris Brezillon
<[email protected]> wrote:
> On Tue, 4 Aug 2026 16:15:49 +0200
> Boris Brezillon <[email protected]> wrote:
> > Actually, if we're making the drm_gem_shmem_record_mkwrite() call
> > unconditional (for PTE and PMD updates) in that path, can't we drop the
> > drm_gem_shmem_pfn_mkwrite() call living in drm_gem_shmem_pfn_mkwrite()?
> >
> > Also, I'm not even sure we can end up with write=true for PTE updates,
> > because our pfn_mkwrite implementation returns zero, not VM_FAULT_ERROR
> > or VM_FAULT_NOPAGE. This means the default RO -> RW PTE upgrade
> > implemented in finish_mkwrite_fault() [1] will take place. If we really
> > want out try_insert_pfn() to be called for those RO -> RW updgrades, we
> > need to call try_insert_pfn() from drm_gem_shmem_pfn_mkwrite().
>
> Nevermind, it's all explained in the comment you've added. Sorry for
> the noise. I keep wondering if we shouldn't call try_insert_pfn() from
> pfn_mkwrite() though, like is done in other places.

No, the .pfn_mkwrite() callback is invoked when a PTE already exists,
and mm/ already takes care of making it writable. So there's nothing
to insert, you just have to take note which you do with
drm_gem_shmem_record_mkwrite().

Paolo