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

Zhou, Xianrong
2024-01-23T08:33:03Z

[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@amd.com>
> > ---
> > 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 */