Re: [PATCH v2] drm/ttm: Do not put non-struct page memory into PUD/PMDs

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On 10/20/21 01:27, Jason Gunthorpe wrote:
> PUD and PMD entries do not have a special bit.
>
> get_user_pages_fast() considers any page that passed pmd_huge() as
> usable:
>
> 	if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd) ||
> 		     pmd_devmap(pmd))) {
>
> And vmf_insert_pfn_pmd_prot() unconditionally sets
>
> 	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>
> eg on x86 the page will be _PAGE_PRESENT | PAGE_PSE.
>
> As such gup_huge_pmd() will try to deref a struct page:
>
> 	head = try_grab_compound_head(pmd_page(orig), refs, flags);
>
> and thus crash.
>
> Thomas further notices that the drivers are not expecting the struct page
> to be used by anything - in particular the refcount incr above will cause
> them to malfunction.
>
> Thus everything about this is not able to fully work correctly considering
> GUP_fast. Delete it entirely. It can return someday along with a proper
> PMD/PUD_SPECIAL bit in the page table itself to gate GUP_fast.
>
> Fixes: 314b6580adc5 ("drm/ttm, drm/vmwgfx: Support huge TTM pagefaults")
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_gem.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            | 94 +---------------------
>   drivers/gpu/drm/vmwgfx/vmwgfx_drv.h        |  4 -
>   drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c | 72 +----------------
>   drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c   |  3 -
>   include/drm/ttm/ttm_bo_api.h               |  3 +-
>   8 files changed, 7 insertions(+), 175 deletions(-)
>
> v2:
>   - Remove the entire thing as per Thomas's advice
> v1: https://lore.kernel.org/r/0-v1-69e7da97f81f+21c-ttm_pmd_jgg@nvidia.com
>
> After this patch the only users of the vmf_insert_pfn_pud/pmd_prot() functions
> are DAX and DAX always has a struct page. Eliminating this non-working case
> will simplify trying to fix the refcounting on ZONE_DEVICE pages.
>
> Thanks,
> Jason

I think the patch subject needs updating to reflect that we're disabling 
PUD/PMDs completely.
With that fixed,

Reviewed-by: Thomas Hellström <thomas.helllstrom@linux.intel.com>

Follow up question: If we resurrect this in the proper way (and in that 
case only for x86_64) is there something we need to pay particular 
attention to WRT the ZONE_DEVICE refcounting fixing you mention above?

Thanks,

Thomas


> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> index d6aa032890ee8b..a1e63ba4c54a59 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c
> @@ -61,7 +61,7 @@ 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, 1);
> +						TTM_BO_VM_NUM_PREFAULT);
>   
>   		 drm_dev_exit(idx);
>   	} else {
> diff --git a/drivers/gpu/drm/nouveau/nouveau_gem.c b/drivers/gpu/drm/nouveau/nouveau_gem.c
> index 8c2ecc28272322..c89d5964148fd5 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, 1);
> +	ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT);
>   	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 458f92a7088797..a36a4f2c76b097 100644
> --- a/drivers/gpu/drm/radeon/radeon_gem.c
> +++ b/drivers/gpu/drm/radeon/radeon_gem.c
> @@ -61,7 +61,7 @@ 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, 1);
> +				       TTM_BO_VM_NUM_PREFAULT);
>   	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 f56be5bc0861ec..e5af7f9e94b273 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -171,89 +171,6 @@ vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
>   }
>   EXPORT_SYMBOL(ttm_bo_vm_reserve);
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -/**
> - * ttm_bo_vm_insert_huge - Insert a pfn for PUD or PMD faults
> - * @vmf: Fault data
> - * @bo: The buffer object
> - * @page_offset: Page offset from bo start
> - * @fault_page_size: The size of the fault in pages.
> - * @pgprot: The page protections.
> - * Does additional checking whether it's possible to insert a PUD or PMD
> - * pfn and performs the insertion.
> - *
> - * Return: VM_FAULT_NOPAGE on successful insertion, VM_FAULT_FALLBACK if
> - * a huge fault was not possible, or on insertion error.
> - */
> -static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
> -					struct ttm_buffer_object *bo,
> -					pgoff_t page_offset,
> -					pgoff_t fault_page_size,
> -					pgprot_t pgprot)
> -{
> -	pgoff_t i;
> -	vm_fault_t ret;
> -	unsigned long pfn;
> -	pfn_t pfnt;
> -	struct ttm_tt *ttm = bo->ttm;
> -	bool write = vmf->flags & FAULT_FLAG_WRITE;
> -
> -	/* Fault should not cross bo boundary. */
> -	page_offset &= ~(fault_page_size - 1);
> -	if (page_offset + fault_page_size > bo->resource->num_pages)
> -		goto out_fallback;
> -
> -	if (bo->resource->bus.is_iomem)
> -		pfn = ttm_bo_io_mem_pfn(bo, page_offset);
> -	else
> -		pfn = page_to_pfn(ttm->pages[page_offset]);
> -
> -	/* pfn must be fault_page_size aligned. */
> -	if ((pfn & (fault_page_size - 1)) != 0)
> -		goto out_fallback;
> -
> -	/* Check that memory is contiguous. */
> -	if (!bo->resource->bus.is_iomem) {
> -		for (i = 1; i < fault_page_size; ++i) {
> -			if (page_to_pfn(ttm->pages[page_offset + i]) != pfn + i)
> -				goto out_fallback;
> -		}
> -	} else if (bo->bdev->funcs->io_mem_pfn) {
> -		for (i = 1; i < fault_page_size; ++i) {
> -			if (ttm_bo_io_mem_pfn(bo, page_offset + i) != pfn + i)
> -				goto out_fallback;
> -		}
> -	}
> -
> -	pfnt = __pfn_to_pfn_t(pfn, PFN_DEV);
> -	if (fault_page_size == (HPAGE_PMD_SIZE >> PAGE_SHIFT))
> -		ret = vmf_insert_pfn_pmd_prot(vmf, pfnt, pgprot, write);
> -#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> -	else if (fault_page_size == (HPAGE_PUD_SIZE >> PAGE_SHIFT))
> -		ret = vmf_insert_pfn_pud_prot(vmf, pfnt, pgprot, write);
> -#endif
> -	else
> -		WARN_ON_ONCE(ret = VM_FAULT_FALLBACK);
> -
> -	if (ret != VM_FAULT_NOPAGE)
> -		goto out_fallback;
> -
> -	return VM_FAULT_NOPAGE;
> -out_fallback:
> -	count_vm_event(THP_FAULT_FALLBACK);
> -	return VM_FAULT_FALLBACK;
> -}
> -#else
> -static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
> -					struct ttm_buffer_object *bo,
> -					pgoff_t page_offset,
> -					pgoff_t fault_page_size,
> -					pgprot_t pgprot)
> -{
> -	return VM_FAULT_FALLBACK;
> -}
> -#endif
> -
>   /**
>    * ttm_bo_vm_fault_reserved - TTM fault helper
>    * @vmf: The struct vm_fault given as argument to the fault callback
> @@ -261,7 +178,6 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
>    * @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.
> - * @fault_page_size: The size of the fault in pages.
>    *
>    * This function inserts one or more page table entries pointing to the
>    * memory backing the buffer object, and then returns a return code
> @@ -275,8 +191,7 @@ static vm_fault_t ttm_bo_vm_insert_huge(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 fault_page_size)
> +				    pgoff_t num_prefault)
>   {
>   	struct vm_area_struct *vma = vmf->vma;
>   	struct ttm_buffer_object *bo = vma->vm_private_data;
> @@ -327,11 +242,6 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>   		prot = pgprot_decrypted(prot);
>   	}
>   
> -	/* We don't prefault on huge faults. Yet. */
> -	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && fault_page_size != 1)
> -		return ttm_bo_vm_insert_huge(vmf, bo, page_offset,
> -					     fault_page_size, prot);
> -
>   	/*
>   	 * Speculatively prefault a number of pages. Only error on
>   	 * first page.
> @@ -429,7 +339,7 @@ vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
>   
>   	prot = vma->vm_page_prot;
>   	if (drm_dev_enter(ddev, &idx)) {
> -		ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT, 1);
> +		ret = ttm_bo_vm_fault_reserved(vmf, prot, TTM_BO_VM_NUM_PREFAULT);
>   		drm_dev_exit(idx);
>   	} else {
>   		ret = ttm_bo_vm_dummy_page(vmf, prot);
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> index a833751099b559..858aff99a3fe53 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_drv.h
> @@ -1550,10 +1550,6 @@ void vmw_bo_dirty_unmap(struct vmw_buffer_object *vbo,
>   			pgoff_t start, pgoff_t end);
>   vm_fault_t vmw_bo_vm_fault(struct vm_fault *vmf);
>   vm_fault_t vmw_bo_vm_mkwrite(struct vm_fault *vmf);
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -vm_fault_t vmw_bo_vm_huge_fault(struct vm_fault *vmf,
> -				enum page_entry_size pe_size);
> -#endif
>   
>   /* Transparent hugepage support - vmwgfx_thp.c */
>   #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> index e5a9a5cbd01a7c..922317d1acc8a0 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
> @@ -477,7 +477,7 @@ vm_fault_t vmw_bo_vm_fault(struct vm_fault *vmf)
>   	else
>   		prot = vm_get_page_prot(vma->vm_flags);
>   
> -	ret = ttm_bo_vm_fault_reserved(vmf, prot, num_prefault, 1);
> +	ret = ttm_bo_vm_fault_reserved(vmf, prot, num_prefault);
>   	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
>   		return ret;
>   
> @@ -486,73 +486,3 @@ vm_fault_t vmw_bo_vm_fault(struct vm_fault *vmf)
>   
>   	return ret;
>   }
> -
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -vm_fault_t vmw_bo_vm_huge_fault(struct vm_fault *vmf,
> -				enum page_entry_size pe_size)
> -{
> -	struct vm_area_struct *vma = vmf->vma;
> -	struct ttm_buffer_object *bo = (struct ttm_buffer_object *)
> -	    vma->vm_private_data;
> -	struct vmw_buffer_object *vbo =
> -		container_of(bo, struct vmw_buffer_object, base);
> -	pgprot_t prot;
> -	vm_fault_t ret;
> -	pgoff_t fault_page_size;
> -	bool write = vmf->flags & FAULT_FLAG_WRITE;
> -
> -	switch (pe_size) {
> -	case PE_SIZE_PMD:
> -		fault_page_size = HPAGE_PMD_SIZE >> PAGE_SHIFT;
> -		break;
> -#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> -	case PE_SIZE_PUD:
> -		fault_page_size = HPAGE_PUD_SIZE >> PAGE_SHIFT;
> -		break;
> -#endif
> -	default:
> -		WARN_ON_ONCE(1);
> -		return VM_FAULT_FALLBACK;
> -	}
> -
> -	/* Always do write dirty-tracking and COW on PTE level. */
> -	if (write && (READ_ONCE(vbo->dirty) || is_cow_mapping(vma->vm_flags)))
> -		return VM_FAULT_FALLBACK;
> -
> -	ret = ttm_bo_vm_reserve(bo, vmf);
> -	if (ret)
> -		return ret;
> -
> -	if (vbo->dirty) {
> -		pgoff_t allowed_prefault;
> -		unsigned long page_offset;
> -
> -		page_offset = vmf->pgoff -
> -			drm_vma_node_start(&bo->base.vma_node);
> -		if (page_offset >= bo->resource->num_pages ||
> -		    vmw_resources_clean(vbo, page_offset,
> -					page_offset + PAGE_SIZE,
> -					&allowed_prefault)) {
> -			ret = VM_FAULT_SIGBUS;
> -			goto out_unlock;
> -		}
> -
> -		/*
> -		 * Write protect, so we get a new fault on write, and can
> -		 * split.
> -		 */
> -		prot = vm_get_page_prot(vma->vm_flags & ~VM_SHARED);
> -	} else {
> -		prot = vm_get_page_prot(vma->vm_flags);
> -	}
> -
> -	ret = ttm_bo_vm_fault_reserved(vmf, prot, 1, fault_page_size);
> -	if (ret == VM_FAULT_RETRY && !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT))
> -		return ret;
> -
> -out_unlock:
> -	dma_resv_unlock(bo->base.resv);
> -
> -	return ret;
> -}
> -#endif
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> index e6b1f98ec99f09..0a4c340252ec4a 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_glue.c
> @@ -61,9 +61,6 @@ int vmw_mmap(struct file *filp, struct vm_area_struct *vma)
>   		.fault = vmw_bo_vm_fault,
>   		.open = ttm_bo_vm_open,
>   		.close = ttm_bo_vm_close,
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -		.huge_fault = vmw_bo_vm_huge_fault,
> -#endif
>   	};
>   	struct drm_file *file_priv = filp->private_data;
>   	struct vmw_private *dev_priv = vmw_priv(file_priv->minor->dev);
> diff --git a/include/drm/ttm/ttm_bo_api.h b/include/drm/ttm/ttm_bo_api.h
> index f681bbdbc6982e..36f7eb9d066395 100644
> --- a/include/drm/ttm/ttm_bo_api.h
> +++ b/include/drm/ttm/ttm_bo_api.h
> @@ -594,8 +594,7 @@ vm_fault_t ttm_bo_vm_reserve(struct ttm_buffer_object *bo,
>   
>   vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
>   				    pgprot_t prot,
> -				    pgoff_t num_prefault,
> -				    pgoff_t fault_page_size);
> +				    pgoff_t num_prefault);
>   
>   vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf);
>   
>
> base-commit: 519d81956ee277b4419c723adfb154603c2565ba