drm/ttm: rework ttm_tt page limit v4

TTM implements a rather extensive accounting of allocated memory.

There are two reasons for this:
1. It tries to block userspace allocating a huge number of very small
   BOs without accounting for the kmalloced memory.

2. Make sure we don't over allocate and run into an OOM situation
   during swapout while trying to handle the memory shortage.

This is only partially a good idea. First of all it is perfectly
valid for an application to use all of system memory, limiting it to
50% is not really acceptable.

What we need to take care of is that the application is held
accountable for the memory it allocated. This is what control
mechanisms like memcg and the normal Linux page accounting already do.

Making sure that we don't run into an OOM situation while trying to
cope with a memory shortage is still a good idea, but this is also
not very well implemented since it means another opportunity of
recursion from the driver back into TTM.

So start to rework all of this by implementing a shrinker callback which
allows for TT object to be swapped out if necessary.

v2: Switch from limit to shrinker callback.
v3: fix gfp mask handling, use atomic for swapable_pages, add debugfs
v4: drop the extra gfp_mask checks
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210208133226.36955-1-christian.koenig@amd.com

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1219,7 +1219,7 @@ EXPORT_SYMBOL(ttm_bo_wait);
  * A buffer object shrink method that tries to swap out the first
  * buffer object on the bo_global::swap_lru list.
  */
-int ttm_bo_swapout(struct ttm_operation_ctx *ctx)
+int ttm_bo_swapout(struct ttm_operation_ctx *ctx, gfp_t gfp_flags)
 {
 	struct ttm_global *glob = &ttm_glob;
 	struct ttm_buffer_object *bo;
@@ -1302,7 +1302,7 @@ int ttm_bo_swapout(struct ttm_operation_ctx *ctx)
 	if (bo->bdev->funcs->swap_notify)
 		bo->bdev->funcs->swap_notify(bo);
 
-	ret = ttm_tt_swapout(bo->bdev, bo->ttm);
+	ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
 out:
 
 	/**

drivers/gpu/drm/ttm/ttm_memory.c

@@ -37,6 +37,7 @@
 #include <linux/slab.h>
 #include <linux/swap.h>
 #include <drm/ttm/ttm_pool.h>
+#include <drm/ttm/ttm_tt.h>
 
 #include "ttm_module.h"
 
@@ -276,9 +277,9 @@ static void ttm_shrink(struct ttm_mem_global *glob, bool from_wq,
 
 	while (ttm_zones_above_swap_target(glob, from_wq, extra)) {
 		spin_unlock(&glob->lock);
-		ret = ttm_bo_swapout(ctx);
+		ret = ttm_bo_swapout(ctx, GFP_KERNEL);
 		spin_lock(&glob->lock);
-		if (unlikely(ret != 0))
+		if (unlikely(ret < 0))
 			break;
 	}
 
@@ -453,6 +454,7 @@ int ttm_mem_global_init(struct ttm_mem_global *glob)
 			zone->name, (unsigned long long)zone->max_mem >> 10);
 	}
 	ttm_pool_mgr_init(glob->zone_kernel->max_mem/(2*PAGE_SIZE));
+	ttm_tt_mgr_init();
 	return 0;
 out_no_zone:
 	ttm_mem_global_release(glob);
@@ -466,6 +468,7 @@ void ttm_mem_global_release(struct ttm_mem_global *glob)
 
 	/* let the page allocator first stop the shrink work. */
 	ttm_pool_mgr_fini();
+	ttm_tt_mgr_fini();
 
 	flush_workqueue(glob->swap_queue);
 	destroy_workqueue(glob->swap_queue);

drivers/gpu/drm/ttm/ttm_tt.c

@@ -38,6 +38,11 @@
 #include <drm/drm_cache.h>
 #include <drm/ttm/ttm_bo_driver.h>
 
+#include "ttm_module.h"
+
+static struct shrinker mm_shrinker;
+static atomic_long_t swapable_pages;
+
 /*
  * Allocates a ttm structure for the given BO.
  */
@@ -223,32 +228,41 @@ int ttm_tt_swapin(struct ttm_tt *ttm)
 	return ret;
 }
 
-int ttm_tt_swapout(struct ttm_device *bdev, struct ttm_tt *ttm)
+/**
+ * ttm_tt_swapout - swap out tt object
+ *
+ * @bdev: TTM device structure.
+ * @ttm: The struct ttm_tt.
+ * @gfp_flags: Flags to use for memory allocation.
+ *
+ * Swapout a TT object to a shmem_file, return number of pages swapped out or
+ * negative error code.
+ */
+int ttm_tt_swapout(struct ttm_device *bdev, struct ttm_tt *ttm,
+		   gfp_t gfp_flags)
 {
+	loff_t size = (loff_t)ttm->num_pages << PAGE_SHIFT;
 	struct address_space *swap_space;
 	struct file *swap_storage;
 	struct page *from_page;
 	struct page *to_page;
-	gfp_t gfp_mask;
 	int i, ret;
 
-	swap_storage = shmem_file_setup("ttm swap",
-					ttm->num_pages << PAGE_SHIFT,
-					0);
+	swap_storage = shmem_file_setup("ttm swap", size, 0);
 	if (IS_ERR(swap_storage)) {
 		pr_err("Failed allocating swap storage\n");
 		return PTR_ERR(swap_storage);
 	}
 
 	swap_space = swap_storage->f_mapping;
-	gfp_mask = mapping_gfp_mask(swap_space);
+	gfp_flags &= mapping_gfp_mask(swap_space);
 
 	for (i = 0; i < ttm->num_pages; ++i) {
 		from_page = ttm->pages[i];
 		if (unlikely(from_page == NULL))
 			continue;
 
-		to_page = shmem_read_mapping_page_gfp(swap_space, i, gfp_mask);
+		to_page = shmem_read_mapping_page_gfp(swap_space, i, gfp_flags);
 		if (IS_ERR(to_page)) {
 			ret = PTR_ERR(to_page);
 			goto out_err;
@@ -263,7 +277,7 @@ int ttm_tt_swapout(struct ttm_device *bdev, struct ttm_tt *ttm)
 	ttm->swap_storage = swap_storage;
 	ttm->page_flags |= TTM_PAGE_FLAG_SWAPPED;
 
-	return 0;
+	return ttm->num_pages;
 
 out_err:
 	fput(swap_storage);
@@ -280,6 +294,8 @@ static void ttm_tt_add_mapping(struct ttm_device *bdev, struct ttm_tt *ttm)
 
 	for (i = 0; i < ttm->num_pages; ++i)
 		ttm->pages[i]->mapping = bdev->dev_mapping;
+
+	atomic_long_add(ttm->num_pages, &swapable_pages);
 }
 
 int ttm_tt_populate(struct ttm_device *bdev,
@@ -326,6 +342,8 @@ static void ttm_tt_clear_mapping(struct ttm_tt *ttm)
 		(*page)->mapping = NULL;
 		(*page++)->index = 0;
 	}
+
+	atomic_long_sub(ttm->num_pages, &swapable_pages);
 }
 
 void ttm_tt_unpopulate(struct ttm_device *bdev,
@@ -341,3 +359,74 @@ void ttm_tt_unpopulate(struct ttm_device *bdev,
 		ttm_pool_free(&bdev->pool, ttm);
 	ttm->page_flags &= ~TTM_PAGE_FLAG_PRIV_POPULATED;
 }
+
+/* As long as pages are available make sure to release at least one */
+static unsigned long ttm_tt_shrinker_scan(struct shrinker *shrink,
+					  struct shrink_control *sc)
+{
+	struct ttm_operation_ctx ctx = {
+		.no_wait_gpu = false
+	};
+	int ret;
+
+	ret = ttm_bo_swapout(&ctx, GFP_NOFS);
+	return ret < 0 ? SHRINK_EMPTY : ret;
+}
+
+/* Return the number of pages available or SHRINK_EMPTY if we have none */
+static unsigned long ttm_tt_shrinker_count(struct shrinker *shrink,
+					   struct shrink_control *sc)
+{
+	unsigned long num_pages;
+
+	num_pages = atomic_long_read(&swapable_pages);
+	return num_pages ? num_pages : SHRINK_EMPTY;
+}
+
+#ifdef CONFIG_DEBUG_FS
+
+/* Test the shrinker functions and dump the result */
+static int ttm_tt_debugfs_shrink_show(struct seq_file *m, void *data)
+{
+	struct shrink_control sc = { .gfp_mask = GFP_KERNEL };
+
+	fs_reclaim_acquire(GFP_KERNEL);
+	seq_printf(m, "%lu/%lu\n", ttm_tt_shrinker_count(&mm_shrinker, &sc),
+		   ttm_tt_shrinker_scan(&mm_shrinker, &sc));
+	fs_reclaim_release(GFP_KERNEL);
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(ttm_tt_debugfs_shrink);
+
+#endif
+
+
+
+/**
+ * ttm_tt_mgr_init - register with the MM shrinker
+ *
+ * Register with the MM shrinker for swapping out BOs.
+ */
+int ttm_tt_mgr_init(void)
+{
+#ifdef CONFIG_DEBUG_FS
+	debugfs_create_file("tt_shrink", 0400, ttm_debugfs_root, NULL,
+			    &ttm_tt_debugfs_shrink_fops);
+#endif
+
+	mm_shrinker.count_objects = ttm_tt_shrinker_count;
+	mm_shrinker.scan_objects = ttm_tt_shrinker_scan;
+	mm_shrinker.seeks = 1;
+	return register_shrinker(&mm_shrinker);
+}
+
+/**
+ * ttm_tt_mgr_fini - unregister our MM shrinker
+ *
+ * Unregisters the MM shrinker.
+ */
+void ttm_tt_mgr_fini(void)
+{
+	unregister_shrinker(&mm_shrinker);
+}

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -1383,7 +1383,7 @@ static int vmw_pm_freeze(struct device *kdev)
 	vmw_execbuf_release_pinned_bo(dev_priv);
 	vmw_resource_evict_all(dev_priv);
 	vmw_release_device_early(dev_priv);
-	while (ttm_bo_swapout(&ctx) == 0);
+	while (ttm_bo_swapout(&ctx, GFP_KERNEL) > 0);
 	if (dev_priv->enable_fb)
 		vmw_fifo_resource_dec(dev_priv);
 	if (atomic_read(&dev_priv->num_fifo_resources) != 0) {

include/drm/ttm/ttm_bo_api.h

@@ -569,7 +569,7 @@ ssize_t ttm_bo_io(struct ttm_device *bdev, struct file *filp,
 		  const char __user *wbuf, char __user *rbuf,
 		  size_t count, loff_t *f_pos, bool write);
 
-int ttm_bo_swapout(struct ttm_operation_ctx *ctx);
+int ttm_bo_swapout(struct ttm_operation_ctx *ctx, gfp_t gfp_flags);
 
 /**
  * ttm_bo_uses_embedded_gem_object - check if the given bo uses the

include/drm/ttm/ttm_tt.h

@@ -135,7 +135,8 @@ void ttm_tt_destroy_common(struct ttm_device *bdev, struct ttm_tt *ttm);
  * Swap in a previously swap out ttm_tt.
  */
 int ttm_tt_swapin(struct ttm_tt *ttm);
-int ttm_tt_swapout(struct ttm_device *bdev, struct ttm_tt *ttm);
+int ttm_tt_swapout(struct ttm_device *bdev, struct ttm_tt *ttm,
+		   gfp_t gfp_flags);
 
 /**
  * ttm_tt_populate - allocate pages for a ttm
@@ -155,6 +156,9 @@ int ttm_tt_populate(struct ttm_device *bdev, struct ttm_tt *ttm, struct ttm_oper
  */
 void ttm_tt_unpopulate(struct ttm_device *bdev, struct ttm_tt *ttm);
 
+int ttm_tt_mgr_init(void);
+void ttm_tt_mgr_fini(void);
+
 #if IS_ENABLED(CONFIG_AGP)
 #include <linux/agp_backend.h>