drm/vgem: Pin our pages for dmabuf exports

When the caller maps their dmabuf and we return an sg_table, the caller
doesn't expect the pages beneath that sg_table to vanish on a whim (i.e.
under mempressure). The contract is that the pages are pinned for the
duration of the mapping (from dma_buf_map_attachment() to
dma_buf_unmap_attachment). To comply, we need to introduce our own
vgem_object.pages_pin_count and elevate it across the mapping. However,
the drm_prime interface we use calls drv->prime_pin on dma_buf_attach
and drv->prime_unpin on dma_buf_detach, which while that does cover the
mapping is much broader than is desired -- but it will do for now.

v2: also hold the pin across prime_vmap/vunmap

Reported-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Testcase: igt/gem_concurrent_blit/*swap*vgem*
Fixes: 5ba6c9ff961a ("drm/vgem: Fix mmaping")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: <stable@vger.kernel.org> # needs a backport
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622134617.17912-1-chris@chris-wilson.co.uk

drivers/gpu/drm/vgem/vgem_drv.c

@@ -52,6 +52,7 @@ static void vgem_gem_free_object(struct drm_gem_object *obj)
 	struct drm_vgem_gem_object *vgem_obj = to_vgem_bo(obj);
 
 	kvfree(vgem_obj->pages);
+	mutex_destroy(&vgem_obj->pages_lock);
 
 	if (obj->import_attach)
 		drm_prime_gem_destroy(obj, vgem_obj->table);
@@ -76,11 +77,15 @@ static int vgem_gem_fault(struct vm_fault *vmf)
 	if (page_offset > num_pages)
 		return VM_FAULT_SIGBUS;
 
+	ret = -ENOENT;
+	mutex_lock(&obj->pages_lock);
 	if (obj->pages) {
 		get_page(obj->pages[page_offset]);
 		vmf->page = obj->pages[page_offset];
 		ret = 0;
-	} else {
+	}
+	mutex_unlock(&obj->pages_lock);
+	if (ret) {
 		struct page *page;
 
 		page = shmem_read_mapping_page(
@@ -161,6 +166,8 @@ static struct drm_vgem_gem_object *__vgem_gem_create(struct drm_device *dev,
 		return ERR_PTR(ret);
 	}
 
+	mutex_init(&obj->pages_lock);
+
 	return obj;
 }
 
@@ -274,37 +281,66 @@ static const struct file_operations vgem_driver_fops = {
 	.release	= drm_release,
 };
 
+static struct page **vgem_pin_pages(struct drm_vgem_gem_object *bo)
+{
+	mutex_lock(&bo->pages_lock);
+	if (bo->pages_pin_count++ == 0) {
+		struct page **pages;
+
+		pages = drm_gem_get_pages(&bo->base);
+		if (IS_ERR(pages)) {
+			bo->pages_pin_count--;
+			mutex_unlock(&bo->pages_lock);
+			return pages;
+		}
+
+		bo->pages = pages;
+	}
+	mutex_unlock(&bo->pages_lock);
+
+	return bo->pages;
+}
+
+static void vgem_unpin_pages(struct drm_vgem_gem_object *bo)
+{
+	mutex_lock(&bo->pages_lock);
+	if (--bo->pages_pin_count == 0) {
+		drm_gem_put_pages(&bo->base, bo->pages, true, true);
+		bo->pages = NULL;
+	}
+	mutex_unlock(&bo->pages_lock);
+}
+
 static int vgem_prime_pin(struct drm_gem_object *obj)
 {
+	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
 	long n_pages = obj->size >> PAGE_SHIFT;
 	struct page **pages;
 
-	/* Flush the object from the CPU cache so that importers can rely
-	 * on coherent indirect access via the exported dma-address.
-	 */
-	pages = drm_gem_get_pages(obj);
+	pages = vgem_pin_pages(bo);
 	if (IS_ERR(pages))
 		return PTR_ERR(pages);
 
+	/* Flush the object from the CPU cache so that importers can rely
+	 * on coherent indirect access via the exported dma-address.
+	 */
 	drm_clflush_pages(pages, n_pages);
-	drm_gem_put_pages(obj, pages, true, false);
 
 	return 0;
 }
 
-static struct sg_table *vgem_prime_get_sg_table(struct drm_gem_object *obj)
+static void vgem_prime_unpin(struct drm_gem_object *obj)
 {
-	struct sg_table *st;
-	struct page **pages;
+	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
 
-	pages = drm_gem_get_pages(obj);
-	if (IS_ERR(pages))
-		return ERR_CAST(pages);
+	vgem_unpin_pages(bo);
+}
 
-	st = drm_prime_pages_to_sg(pages, obj->size >> PAGE_SHIFT);
-	drm_gem_put_pages(obj, pages, false, false);
+static struct sg_table *vgem_prime_get_sg_table(struct drm_gem_object *obj)
+{
+	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
 
-	return st;
+	return drm_prime_pages_to_sg(bo->pages, bo->base.size >> PAGE_SHIFT);
 }
 
 static struct drm_gem_object* vgem_prime_import(struct drm_device *dev,
@@ -333,6 +369,8 @@ static struct drm_gem_object *vgem_prime_import_sg_table(struct drm_device *dev,
 		__vgem_gem_destroy(obj);
 		return ERR_PTR(-ENOMEM);
 	}
+
+	obj->pages_pin_count++; /* perma-pinned */
 	drm_prime_sg_to_page_addr_arrays(obj->table, obj->pages, NULL,
 					npages);
 	return &obj->base;
@@ -340,23 +378,23 @@ static struct drm_gem_object *vgem_prime_import_sg_table(struct drm_device *dev,
 
 static void *vgem_prime_vmap(struct drm_gem_object *obj)
 {
+	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
 	long n_pages = obj->size >> PAGE_SHIFT;
 	struct page **pages;
-	void *addr;
 
-	pages = drm_gem_get_pages(obj);
+	pages = vgem_pin_pages(bo);
 	if (IS_ERR(pages))
 		return NULL;
 
-	addr = vmap(pages, n_pages, 0, pgprot_writecombine(PAGE_KERNEL));
-	drm_gem_put_pages(obj, pages, false, false);
-
-	return addr;
+	return vmap(pages, n_pages, 0, pgprot_writecombine(PAGE_KERNEL));
 }
 
 static void vgem_prime_vunmap(struct drm_gem_object *obj, void *vaddr)
 {
+	struct drm_vgem_gem_object *bo = to_vgem_bo(obj);
+
 	vunmap(vaddr);
+	vgem_unpin_pages(bo);
 }
 
 static int vgem_prime_mmap(struct drm_gem_object *obj,
@@ -409,6 +447,7 @@ static struct drm_driver vgem_driver = {
 	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
 	.prime_fd_to_handle = drm_gem_prime_fd_to_handle,
 	.gem_prime_pin = vgem_prime_pin,
+	.gem_prime_unpin = vgem_prime_unpin,
 	.gem_prime_import = vgem_prime_import,
 	.gem_prime_export = drm_gem_prime_export,
 	.gem_prime_import_sg_table = vgem_prime_import_sg_table,

drivers/gpu/drm/vgem/vgem_drv.h

@@ -43,7 +43,11 @@ struct vgem_file {
 #define to_vgem_bo(x) container_of(x, struct drm_vgem_gem_object, base)
 struct drm_vgem_gem_object {
 	struct drm_gem_object base;
+
 	struct page **pages;
+	unsigned int pages_pin_count;
+	struct mutex pages_lock;
+
 	struct sg_table *table;
 };