[RFC PATCH 2/2] mm,drm/ttm: Use VM_PFNMAP for TTM vmas

[Thomas Hellström (Intel)]

To block fast gup we need to make sure TTM ptes are always special.
With MIXEDMAP we, on architectures that don't support pte_special,
insert normal ptes, but OTOH on those architectures, fast is not
supported.
At the same time, the function documentation to vm_normal_page() suggests
that ptes pointing to system memory pages of MIXEDMAP vmas are always
normal, but that doesn't seem consistent with what's implemented in
vmf_insert_mixed(). I'm thus not entirely sure this patch is actually
needed.

But to make sure and to avoid also normal (non-fast) gup, make all
TTM vmas PFNMAP. With PFNMAP we can't allow COW mappings
anymore so make is_cow_mapping() available and use it to reject
COW mappigs at mmap time.

There was previously a comment in the code that WC mappings together
with x86 PAT + PFNMAP was bad for performance. However from looking at
vmf_insert_mixed() it looks like in the current code PFNMAP and MIXEDMAP
are handled the same for architectures that support pte_special. This
means there should not be a performance difference anymore, but this
needs to be verified.

Cc: Christian Koenig <christian.koenig@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Hellström (Intel) <thomas_os@xxxxxxxxxxxx>
---
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 22 ++++++++--------------
 include/linux/mm.h              |  5 +++++
 mm/internal.h                   |  5 -----
 3 files changed, 13 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 1c34983480e5..708c6fb9be81 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -372,12 +372,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf,
 		 * at arbitrary times while the data is mmap'ed.
 		 * See vmf_insert_mixed_prot() for a discussion.
 		 */
-		if (vma->vm_flags & VM_MIXEDMAP)
-			ret = vmf_insert_mixed_prot(vma, address,
-						    __pfn_to_pfn_t(pfn, PFN_DEV),
-						    prot);
-		else
-			ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
+		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
 
 		/* Never error on prefaulted PTEs */
 		if (unlikely((ret & VM_FAULT_ERROR))) {
@@ -555,18 +550,14 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s
 	 * Note: We're transferring the bo reference to
 	 * vma->vm_private_data here.
 	 */
-
 	vma->vm_private_data = bo;
 
 	/*
-	 * We'd like to use VM_PFNMAP on shared mappings, where
-	 * (vma->vm_flags & VM_SHARED) != 0, for performance reasons,
-	 * but for some reason VM_PFNMAP + x86 PAT + write-combine is very
-	 * bad for performance. Until that has been sorted out, use
-	 * VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
+	 * PFNMAP forces us to block COW mappings in mmap(),
+	 * and with MIXEDMAP we would incorrectly allow fast gup
+	 * on TTM memory on architectures that don't have pte_special.
 	 */
-	vma->vm_flags |= VM_MIXEDMAP;
-	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
+	vma->vm_flags |= VM_PFNMAP | VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
 }
 
 int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma,
@@ -579,6 +570,9 @@ int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma,
 	if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET_START))
 		return -EINVAL;
 
+	if (unlikely(is_cow_mapping(vma->vm_flags)))
+		return -EINVAL;
+
 	bo = ttm_bo_vm_lookup(bdev, vma->vm_pgoff, vma_pages(vma));
 	if (unlikely(!bo))
 		return -EINVAL;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 77e64e3eac80..c6ebf7f9ddbb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -686,6 +686,11 @@ static inline bool vma_is_accessible(struct vm_area_struct *vma)
 	return vma->vm_flags & VM_ACCESS_FLAGS;
 }
 
+static inline bool is_cow_mapping(vm_flags_t flags)
+{
+	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
+}
+
 #ifdef CONFIG_SHMEM
 /*
  * The vma_is_shmem is not inline because it is used only by slow
diff --git a/mm/internal.h b/mm/internal.h
index 9902648f2206..1432feec62df 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -296,11 +296,6 @@ static inline unsigned int buddy_order(struct page *page)
  */
 #define buddy_order_unsafe(page)	READ_ONCE(page_private(page))
 
-static inline bool is_cow_mapping(vm_flags_t flags)
-{
-	return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
-}
-
 /*
  * These three helpers classifies VMAs for virtual memory accounting.
  */
-- 
2.30.2