RE: [PATCH] mm: Remove double faults once write a device pfn

[Zhou, Xianrong]

[AMD Official Use Only - General]

> > The vmf_insert_pfn_prot could cause unnecessary double faults on a
> > device pfn. Because currently the vmf_insert_pfn_prot does not make
> > the pfn writable so the pte entry is normally read-only or dirty
> > catching.
>
> What? How do you got to this conclusion?

Sorry. I did not mention that this problem only exists on arm64 platform.
Because on arm64 platform the PTE_RDONLY is automatically attached to
the userspace pte entries even through VM_WRITE + VM_SHARE.
The  PTE_RDONLY needs to be cleared in vmf_insert_pfn_prot. However
vmf_insert_pfn_prot do not make the pte writable passing false @mkwrite
to insert_pfn.

>
> > The first fault only sets up the pte entry which actually is dirty
> > catching. And the second immediate fault to the pfn due to first dirty
> > catching when the cpu re-execute the store instruction.
>
> It could be that this is done to work around some hw behavior, but not
> because of dirty catching.
>
> > Normally if the drivers call vmf_insert_pfn_prot and also supply
> > 'pfn_mkwrite' callback within vm_operations_struct which requires the
> > pte to be dirty catching then the vmf_insert_pfn_prot and the double
> > fault are reasonable. It is not a problem.
>
> Well, as far as I can see that behavior absolutely doesn't make sense.
>
> When pfn_mkwrite is requested then the driver should use PAGE_COPY, which
> is exactly what VMWGFX (the only driver using dirty tracking) is doing.
>
> Everybody else uses PAGE_SHARED which should make the pte writeable
> immediately.
>
> Regards,
> Christian.
>
> >
> > However the most of drivers calling vmf_insert_pfn_prot do not supply
> > the 'pfn_mkwrite' callback so that the second fault is unnecessary.
> >
> > So just like vmf_insert_mixed and vmf_insert_mixed_mkwrite pair, we
> > should also supply vmf_insert_pfn_mkwrite for drivers as well.
> >
> > Signed-off-by: Xianrong Zhou <Xianrong.Zhou@xxxxxxx>
> > ---
> >   arch/x86/entry/vdso/vma.c                  |  3 ++-
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c    |  2 +-
> >   drivers/gpu/drm/i915/gem/i915_gem_ttm.c    |  2 +-
> >   drivers/gpu/drm/nouveau/nouveau_gem.c      |  2 +-
> >   drivers/gpu/drm/radeon/radeon_gem.c        |  2 +-
> >   drivers/gpu/drm/ttm/ttm_bo_vm.c            |  8 +++++---
> >   drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c |  8 +++++---
> >   include/drm/ttm/ttm_bo.h                   |  3 ++-
> >   include/linux/mm.h                         |  2 +-
> >   mm/memory.c                                | 14 +++++++++++---
> >   10 files changed, 30 insertions(+), 16 deletions(-)
> >
> > diff --git a/arch/x86/entry/vdso/vma.c b/arch/x86/entry/vdso/vma.c
> > index 7645730dc228..dd2431c2975f 100644
> > --- a/arch/x86/entry/vdso/vma.c
> > +++ b/arch/x86/entry/vdso/vma.c
> > @@ -185,7 +185,8 @@ static vm_fault_t vvar_fault(const struct
> vm_special_mapping *sm,
> >             if (pvti && vclock_was_used(VDSO_CLOCKMODE_PVCLOCK))
> {
> >                     return vmf_insert_pfn_prot(vma, vmf->address,
> >                                     __pa(pvti) >> PAGE_SHIFT,
> > -                                   pgprot_decrypted(vma-
> >vm_page_prot));
> > +                                   pgprot_decrypted(vma-
> >vm_page_prot),
> > +                                   true);
> >             }
> >     } else if (sym_offset == image->sym_hvclock_page) {
> >             pfn = hv_get_tsc_pfn();
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > index 49a5f1c73b3e..adcb20d9e624 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> > @@ -64,7 +64,7 @@ static vm_fault_t amdgpu_gem_fault(struct vm_fault
> *vmf)
> >             }
> >
> >             ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma-
> >vm_page_prot,
> > -                                          TTM_BO_VM_NUM_PREFAULT);
> > +                                          TTM_BO_VM_NUM_PREFAULT,
> true);
> >
> >             drm_dev_exit(idx);
> >     } else {
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > index 9227f8146a58..c6f13ae6c308 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > @@ -1114,7 +1114,7 @@ static vm_fault_t vm_fault_ttm(struct vm_fault
> > *vmf)
> >
> >     if (drm_dev_enter(dev, &idx)) {
> >             ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma-
> >vm_page_prot,
> > -                                          TTM_BO_VM_NUM_PREFAULT);
> > +                                          TTM_BO_VM_NUM_PREFAULT,
> true);
> >             drm_dev_exit(idx);
> >     } else {
> >             ret = ttm_bo_vm_dummy_page(vmf, vmf->vma-
> >vm_page_prot); diff
> > --git a/drivers/gpu/drm/nouveau/nouveau_gem.c
> > b/drivers/gpu/drm/nouveau/nouveau_gem.c
> > index 49c2bcbef129..7e1453762ec9 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_gem.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_gem.c
> > @@ -56,7 +56,7 @@ static vm_fault_t nouveau_ttm_fault(struct vm_fault
> > *vmf)
> >
> >     nouveau_bo_del_io_reserve_lru(bo);
> >     prot = vm_get_page_prot(vma->vm_flags);
> > -   ret = ttm_bo_vm_fault_reserved(vmf, prot,
> TTM_BO_VM_NUM_PREFAULT);
> > +   ret = ttm_bo_vm_fault_reserved(vmf, prot,
> TTM_BO_VM_NUM_PREFAULT,
> > +true);
> >     nouveau_bo_add_io_reserve_lru(bo);
> >     if (ret == VM_FAULT_RETRY && !(vmf->flags &
> FAULT_FLAG_RETRY_NOWAIT))
> >             return ret;
> > diff --git a/drivers/gpu/drm/radeon/radeon_gem.c
> > b/drivers/gpu/drm/radeon/radeon_gem.c
> > index 3fec3acdaf28..b21cf00ae162 100644
> > --- a/drivers/gpu/drm/radeon/radeon_gem.c
> > +++ b/drivers/gpu/drm/radeon/radeon_gem.c
> > @@ -62,7 +62,7 @@ static vm_fault_t radeon_gem_fault(struct vm_fault
> *vmf)
> >             goto unlock_resv;
> >
> >     ret = ttm_bo_vm_fault_reserved(vmf, vmf->vma->vm_page_prot,
> > -                                  TTM_BO_VM_NUM_PREFAULT);
> > +                                  TTM_BO_VM_NUM_PREFAULT, true);
> >     if (ret == VM_FAULT_RETRY && !(vmf->flags &
> FAULT_FLAG_RETRY_NOWAIT))
> >             goto unlock_mclk;
> >
> > diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > b/drivers/gpu/drm/ttm/ttm_bo_vm.c index
> 4212b8c91dd4..7d14a7d267aa
> > 100644
> > --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> > @@ -167,6 +167,7 @@ EXPORT_SYMBOL(ttm_bo_vm_reserve);
> >    * @num_prefault: Maximum number of prefault pages. The caller may
> want to
> >    * specify this based on madvice settings and the size of the GPU object
> >    * backed by the memory.
> > + * @mkwrite: make the pfn or page writable
> >    *
> >    * This function inserts one or more page table entries pointing to the
> >    * memory backing the buffer object, and then returns a return code
> > @@ -180,7 +181,8 @@ EXPORT_SYMBOL(ttm_bo_vm_reserve);
> >    */
> >   vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
> >                                 pgprot_t prot,
> > -                               pgoff_t num_prefault)
> > +                               pgoff_t num_prefault,
> > +                               bool mkwrite)
> >   {
> >     struct vm_area_struct *vma = vmf->vma;
> >     struct ttm_buffer_object *bo = vma->vm_private_data; @@ -263,7
> > +265,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
> >              * at arbitrary times while the data is mmap'ed.
> >              * See vmf_insert_pfn_prot() for a discussion.
> >              */
> > -           ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> > +           ret = vmf_insert_pfn_prot(vma, address, pfn, prot, mkwrite);
> >
> >             /* Never error on prefaulted PTEs */
> >             if (unlikely((ret & VM_FAULT_ERROR))) { @@ -312,7 +314,7
> @@
> > vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
> >     /* Prefault the entire VMA range right away to avoid further faults */
> >     for (address = vma->vm_start; address < vma->vm_end;
> >          address += PAGE_SIZE)
> > -           ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
> > +           ret = vmf_insert_pfn_prot(vma, address, pfn, prot, true);
> >
> >     return ret;
> >   }
> > diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> > b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> > index 74ff2812d66a..bb8e4b641681 100644
> > --- a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> > +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> > @@ -452,12 +452,14 @@ vm_fault_t vmw_bo_vm_fault(struct vm_fault
> *vmf)
> >      * sure the page protection is write-enabled so we don't get
> >      * a lot of unnecessary write faults.
> >      */
> > -   if (vbo->dirty && vbo->dirty->method == VMW_BO_DIRTY_MKWRITE)
> > +   if (vbo->dirty && vbo->dirty->method == VMW_BO_DIRTY_MKWRITE)
> {
> >             prot = vm_get_page_prot(vma->vm_flags & ~VM_SHARED);
> > -   else
> > +           ret = ttm_bo_vm_fault_reserved(vmf, prot, num_prefault,
> false);
> > +   } else {
> >             prot = vm_get_page_prot(vma->vm_flags);
> > +           ret = ttm_bo_vm_fault_reserved(vmf, prot, num_prefault,
> true);
> > +   }
> >
> > -   ret = ttm_bo_vm_fault_reserved(vmf, prot, num_prefault);
> >     if (ret == VM_FAULT_RETRY && !(vmf->flags &
> FAULT_FLAG_RETRY_NOWAIT))
> >             return ret;
> >
> > diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h index
> > 0223a41a64b2..66e293db69ee 100644
> > --- a/include/drm/ttm/ttm_bo.h
> > +++ b/include/drm/ttm/ttm_bo.h
> > @@ -386,7 +386,8 @@ vm_fault_t ttm_bo_vm_reserve(struct
> ttm_buffer_object *bo,
> >                          struct vm_fault *vmf);
> >   vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
> >                                 pgprot_t prot,
> > -                               pgoff_t num_prefault);
> > +                               pgoff_t num_prefault,
> > +                               bool mkwrite);
> >   vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf);
> >   void ttm_bo_vm_open(struct vm_area_struct *vma);
> >   void ttm_bo_vm_close(struct vm_area_struct *vma); diff --git
> > a/include/linux/mm.h b/include/linux/mm.h index
> > f5a97dec5169..f8868e28ea04 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3553,7 +3553,7 @@ int vm_map_pages_zero(struct vm_area_struct
> *vma, struct page **pages,
> >   vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long
> addr,
> >                     unsigned long pfn);
> >   vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned
> long addr,
> > -                   unsigned long pfn, pgprot_t pgprot);
> > +                   unsigned long pfn, pgprot_t pgprot, bool mkwrite);
> >   vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long
> addr,
> >                     pfn_t pfn);
> >   vm_fault_t vmf_insert_mixed_mkwrite(struct vm_area_struct *vma, diff
> > --git a/mm/memory.c b/mm/memory.c index 7e1f4849463a..2c28f1a349ff
> > 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2195,6 +2195,7 @@ static vm_fault_t insert_pfn(struct
> vm_area_struct *vma, unsigned long addr,
> >    * @addr: target user address of this page
> >    * @pfn: source kernel pfn
> >    * @pgprot: pgprot flags for the inserted page
> > + * @mkwrite: make the pfn writable
> >    *
> >    * This is exactly like vmf_insert_pfn(), except that it allows drivers
> >    * to override pgprot on a per-page basis.
> > @@ -2223,7 +2224,7 @@ static vm_fault_t insert_pfn(struct
> vm_area_struct *vma, unsigned long addr,
> >    * Return: vm_fault_t value.
> >    */
> >   vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned
> long addr,
> > -                   unsigned long pfn, pgprot_t pgprot)
> > +                   unsigned long pfn, pgprot_t pgprot, bool mkwrite)
> >   {
> >     /*
> >      * Technically, architectures with pte_special can avoid all these
> > @@ -2246,7 +2247,7 @@ vm_fault_t vmf_insert_pfn_prot(struct
> vm_area_struct *vma, unsigned long addr,
> >     track_pfn_insert(vma, &pgprot, __pfn_to_pfn_t(pfn, PFN_DEV));
> >
> >     return insert_pfn(vma, addr, __pfn_to_pfn_t(pfn, PFN_DEV), pgprot,
> > -                   false);
> > +                   mkwrite);
> >   }
> >   EXPORT_SYMBOL(vmf_insert_pfn_prot);
> >
> > @@ -2273,10 +2274,17 @@ EXPORT_SYMBOL(vmf_insert_pfn_prot);
> >   vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long
> addr,
> >                     unsigned long pfn)
> >   {
> > -   return vmf_insert_pfn_prot(vma, addr, pfn, vma->vm_page_prot);
> > +   return vmf_insert_pfn_prot(vma, addr, pfn, vma->vm_page_prot,
> > +false);
> >   }
> >   EXPORT_SYMBOL(vmf_insert_pfn);
> >
> > +vm_fault_t vmf_insert_pfn_mkwrite(struct vm_area_struct *vma, unsigned
> long addr,
> > +                   unsigned long pfn)
> > +{
> > +   return vmf_insert_pfn_prot(vma, addr, pfn, vma->vm_page_prot,
> true);
> > +} EXPORT_SYMBOL(vmf_insert_pfn_mkwrite);
> > +
> >   static bool vm_mixed_ok(struct vm_area_struct *vma, pfn_t pfn)
> >   {
> >     /* these checks mirror the abort conditions in vm_normal_page */