[PATCH] drm/ttm: nuke VM_MIXEDMAP on BO mappings

[Christian König]

We discussed if that is really the right approach for quite a while now, but
digging deeper into a bug report on arm turned out that this is actually
horrible broken right now.

The reason for this is that vmf_insert_mixed_prot() always tries to grab
a reference to the underlaying page on architectures without
ARCH_HAS_PTE_SPECIAL and as far as I can see also enabled GUP.

So nuke using VM_MIXEDMAP here and use VM_PFNMAP instead.

Also set VM_SHARED, not 100% sure if that is needed with VM_PFNMAP, but better
save than sorry.

Signed-off-by: Christian König <christian.koenig at amd.com>
Bugs: https://gitlab.freedesktop.org/drm/amd/-/issues/1606#note_936174
---
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 29 +++++++----------------------
 1 file changed, 7 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 9bd15cb39145..bf86ae849340 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -359,12 +359,7 @@ 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))) {
@@ -411,15 +406,9 @@ vm_fault_t ttm_bo_vm_dummy_page(struct vm_fault *vmf, pgprot_t prot)
 	pfn = page_to_pfn(page);
 
 	/* Prefault the entire VMA range right away to avoid further faults */
-	for (address = vma->vm_start; address < vma->vm_end; address += PAGE_SIZE) {
-
-		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);
-	}
+	for (address = vma->vm_start; address < vma->vm_end;
+	     address += PAGE_SIZE)
+		ret = vmf_insert_pfn_prot(vma, address, pfn, prot);
 
 	return ret;
 }
@@ -576,14 +565,10 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s
 
 	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
+	/* Enforce VM_SHARED here since no driver backend actually supports COW
+	 * on TTM buffer object mappings.
 	 */
-	vma->vm_flags |= VM_MIXEDMAP;
+	vma->vm_flags |= VM_PFNMAP | VM_SHARED;
 	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
 }
 
-- 
2.25.1