drm/i915: Start writeback from the shrinker

When we are called to relieve mempressue via the shrinker, the only way
we can make progress is either by discarding unwanted pages (those
objects that userspace has marked MADV_DONTNEED) or by reclaiming the
dirty objects via swap. As we know that is the only way to make further
progress, we can initiate the writeback as we invalidate the objects.
This means the objects we put onto the inactive anon lru list are
already marked for reclaim+writeback and so will trigger a wait upon the
writeback inside direct reclaim, greatly improving the success rate of
direct reclaim on i915 objects.

The corollary is that we may start a slow swap on opportunistic
mempressure from the likes of the compaction + migration kthreads. This
is limited by those threads only being allowed to shrink idle pages, but
also that if we reactivate the page before it is swapped out by gpu
activity, we only page the cost of repinning the page. The cost is most
felt when an object is reused after mempressure, which hopefully
excludes the latency sensitive tasks (as we are just extending the
impact of swap thrashing to them).

Apparently this is not the first time we've had this idea. Back in
commit 5537252b6b ("drm/i915: Invalidate our pages under memory
pressure") we wanted to start writeback but settled on invalidate after
Hugh Dickins warned us about a possibility of a deadlock within shmemfs
if we started writeback from shrink_slab. Looking at the callchain,
using writeback from i915_gem_shrink should be equivalent to the pageout
also employed by shrink_slab, i.e. it should not be any riskier afaict.

v2: Leave mmapings intact. At this point, the only mmapings of our
objects will be via CPU mmaps on the shmemfs filp, which are
out-of-scope for our LRU tracking. Instead leave those pages to the
inactive anon LRU page list for aging and pageout as normal.

v3: Be selective on which paths trigger writeback, in particular
excluding paths shrinking just to reclaim vm space (e.g. mmap, vmap
reapers) and avoid starting writeback on the entire process space from
within the pm freezer.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108686
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michal Hocko <mhocko@suse.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190420115539.29081-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3007,7 +3007,7 @@ enum i915_mm_subclass { /* lockdep subclass for obj->mm.lock/struct_mutex */
 
 int __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
 				enum i915_mm_subclass subclass);
-void __i915_gem_object_invalidate(struct drm_i915_gem_object *obj);
+void __i915_gem_object_truncate(struct drm_i915_gem_object *obj);
 
 enum i915_map_type {
 	I915_MAP_WB = 0,
@@ -3268,11 +3268,12 @@ unsigned long i915_gem_shrink(struct drm_i915_private *i915,
 			      unsigned long target,
 			      unsigned long *nr_scanned,
 			      unsigned flags);
-#define I915_SHRINK_PURGEABLE 0x1
-#define I915_SHRINK_UNBOUND 0x2
-#define I915_SHRINK_BOUND 0x4
-#define I915_SHRINK_ACTIVE 0x8
-#define I915_SHRINK_VMAPS 0x10
+#define I915_SHRINK_PURGEABLE	BIT(0)
+#define I915_SHRINK_UNBOUND	BIT(1)
+#define I915_SHRINK_BOUND	BIT(2)
+#define I915_SHRINK_ACTIVE	BIT(3)
+#define I915_SHRINK_VMAPS	BIT(4)
+#define I915_SHRINK_WRITEBACK	BIT(5)
 unsigned long i915_gem_shrink_all(struct drm_i915_private *i915);
 void i915_gem_shrinker_register(struct drm_i915_private *i915);
 void i915_gem_shrinker_unregister(struct drm_i915_private *i915);

drivers/gpu/drm/i915/i915_gem.c

@@ -2143,8 +2143,7 @@ i915_gem_mmap_gtt_ioctl(struct drm_device *dev, void *data,
 }
 
 /* Immediately discard the backing storage */
-static void
-i915_gem_object_truncate(struct drm_i915_gem_object *obj)
+void __i915_gem_object_truncate(struct drm_i915_gem_object *obj)
 {
 	i915_gem_object_free_mmap_offset(obj);
 
@@ -2161,28 +2160,6 @@ i915_gem_object_truncate(struct drm_i915_gem_object *obj)
 	obj->mm.pages = ERR_PTR(-EFAULT);
 }
 
-/* Try to discard unwanted pages */
-void __i915_gem_object_invalidate(struct drm_i915_gem_object *obj)
-{
-	struct address_space *mapping;
-
-	lockdep_assert_held(&obj->mm.lock);
-	GEM_BUG_ON(i915_gem_object_has_pages(obj));
-
-	switch (obj->mm.madv) {
-	case I915_MADV_DONTNEED:
-		i915_gem_object_truncate(obj);
-	case __I915_MADV_PURGED:
-		return;
-	}
-
-	if (obj->base.filp == NULL)
-		return;
-
-	mapping = obj->base.filp->f_mapping,
-	invalidate_mapping_pages(mapping, 0, (loff_t)-1);
-}
-
 /*
  * Move pages to appropriate lru and release the pagevec, decrementing the
  * ref count of those pages.
@@ -4023,7 +4000,7 @@ i915_gem_madvise_ioctl(struct drm_device *dev, void *data,
 	/* if the object is no longer attached, discard its backing storage */
 	if (obj->mm.madv == I915_MADV_DONTNEED &&
 	    !i915_gem_object_has_pages(obj))
-		i915_gem_object_truncate(obj);
+		__i915_gem_object_truncate(obj);
 
 	args->retained = obj->mm.madv != __I915_MADV_PURGED;
 	mutex_unlock(&obj->mm.lock);

drivers/gpu/drm/i915/i915_gem_shrinker.c

@@ -114,6 +114,67 @@ static bool unsafe_drop_pages(struct drm_i915_gem_object *obj)
 	return !i915_gem_object_has_pages(obj);
 }
 
+static void __start_writeback(struct drm_i915_gem_object *obj,
+			      unsigned int flags)
+{
+	struct address_space *mapping;
+	struct writeback_control wbc = {
+		.sync_mode = WB_SYNC_NONE,
+		.nr_to_write = SWAP_CLUSTER_MAX,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+		.for_reclaim = 1,
+	};
+	unsigned long i;
+
+	lockdep_assert_held(&obj->mm.lock);
+	GEM_BUG_ON(i915_gem_object_has_pages(obj));
+
+	switch (obj->mm.madv) {
+	case I915_MADV_DONTNEED:
+		__i915_gem_object_truncate(obj);
+	case __I915_MADV_PURGED:
+		return;
+	}
+
+	if (!obj->base.filp)
+		return;
+
+	if (!(flags & I915_SHRINK_WRITEBACK))
+		return;
+
+	/*
+	 * Leave mmapings intact (GTT will have been revoked on unbinding,
+	 * leaving only CPU mmapings around) and add those pages to the LRU
+	 * instead of invoking writeback so they are aged and paged out
+	 * as normal.
+	 */
+	mapping = obj->base.filp->f_mapping;
+
+	/* Begin writeback on each dirty page */
+	for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) {
+		struct page *page;
+
+		page = find_lock_entry(mapping, i);
+		if (!page || xa_is_value(page))
+			continue;
+
+		if (!page_mapped(page) && clear_page_dirty_for_io(page)) {
+			int ret;
+
+			SetPageReclaim(page);
+			ret = mapping->a_ops->writepage(page, &wbc);
+			if (!PageWriteback(page))
+				ClearPageReclaim(page);
+			if (!ret)
+				goto put;
+		}
+		unlock_page(page);
+put:
+		put_page(page);
+	}
+}
+
 /**
  * i915_gem_shrink - Shrink buffer object caches
  * @i915: i915 device
@@ -254,7 +315,7 @@ i915_gem_shrink(struct drm_i915_private *i915,
 				mutex_lock_nested(&obj->mm.lock,
 						  I915_MM_SHRINKER);
 				if (!i915_gem_object_has_pages(obj)) {
-					__i915_gem_object_invalidate(obj);
+					__start_writeback(obj, flags);
 					count += obj->base.size >> PAGE_SHIFT;
 				}
 				mutex_unlock(&obj->mm.lock);
@@ -366,13 +427,15 @@ i915_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
 				&sc->nr_scanned,
 				I915_SHRINK_BOUND |
 				I915_SHRINK_UNBOUND |
-				I915_SHRINK_PURGEABLE);
+				I915_SHRINK_PURGEABLE |
+				I915_SHRINK_WRITEBACK);
 	if (sc->nr_scanned < sc->nr_to_scan)
 		freed += i915_gem_shrink(i915,
 					 sc->nr_to_scan - sc->nr_scanned,
 					 &sc->nr_scanned,
 					 I915_SHRINK_BOUND |
-					 I915_SHRINK_UNBOUND);
+					 I915_SHRINK_UNBOUND |
+					 I915_SHRINK_WRITEBACK);
 	if (sc->nr_scanned < sc->nr_to_scan && current_is_kswapd()) {
 		intel_wakeref_t wakeref;
 
@@ -382,7 +445,8 @@ i915_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
 						 &sc->nr_scanned,
 						 I915_SHRINK_ACTIVE |
 						 I915_SHRINK_BOUND |
-						 I915_SHRINK_UNBOUND);
+						 I915_SHRINK_UNBOUND |
+						 I915_SHRINK_WRITEBACK);
 		}
 	}
 
@@ -404,7 +468,8 @@ i915_gem_shrinker_oom(struct notifier_block *nb, unsigned long event, void *ptr)
 	with_intel_runtime_pm(i915, wakeref)
 		freed_pages += i915_gem_shrink(i915, -1UL, NULL,
 					       I915_SHRINK_BOUND |
-					       I915_SHRINK_UNBOUND);
+					       I915_SHRINK_UNBOUND |
+					       I915_SHRINK_WRITEBACK);
 
 	/* Because we may be allocating inside our own driver, we cannot
 	 * assert that there are no objects with pinned pages that are not