[PATCH] xen/gntdev: Avoid blocking in unmap_grant_pages()

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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unmap_grant_pages() currently waits for the pages to no longer be used.
In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a
deadlock against i915: i915 was waiting for gntdev's MMU notifier to
finish, while gntdev was waiting for i915 to free its pages.  I also
believe this is responsible for various deadlocks I have experienced in
the past.

Avoid these problems by making unmap_grant_pages async.  This requires
making it return void, as any errors will not be available when the
function returns.  Fortunately, the only use of the return value is a
WARN_ON().  Replace this with WARN_ON()s where errors are detected.

Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in use")
Cc: stable@vger.kernel.org
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/xen/gntdev-common.h |  4 ++
 drivers/xen/gntdev.c        | 82 ++++++++++++++++++-------------------
 2 files changed, 45 insertions(+), 41 deletions(-)

diff --git a/drivers/xen/gntdev-common.h b/drivers/xen/gntdev-common.h
index 20d7d059dadb..a6e2805ea2ce 100644
--- a/drivers/xen/gntdev-common.h
+++ b/drivers/xen/gntdev-common.h
@@ -16,6 +16,7 @@
 #include <linux/mmu_notifier.h>
 #include <linux/types.h>
 #include <xen/interface/event_channel.h>
+#include <xen/grant_table.h>
 
 struct gntdev_dmabuf_priv;
 
@@ -73,6 +74,9 @@ struct gntdev_grant_map {
 	/* Needed to avoid allocation in gnttab_dma_free_pages(). */
 	xen_pfn_t *frames;
 #endif
+
+	/* Needed to avoid allocation in __unmap_grant_pages */
+	struct gntab_unmap_queue_data unmap_data;
 };
 
 struct gntdev_grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count,
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 59ffea800079..670d800e4a89 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -62,8 +62,8 @@ MODULE_PARM_DESC(limit,
 
 static int use_ptemod;
 
-static int unmap_grant_pages(struct gntdev_grant_map *map,
-			     int offset, int pages);
+static void unmap_grant_pages(struct gntdev_grant_map *map,
+			      int offset, int pages);
 
 static struct miscdevice gntdev_miscdev;
 
@@ -349,61 +349,65 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
 	return err;
 }
 
-static int __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
-			       int pages)
+static void __unmap_grant_pages_done(int result,
+		struct gntab_unmap_queue_data *data)
 {
-	int i, err = 0;
-	struct gntab_unmap_queue_data unmap_data;
-
-	if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
-		int pgno = (map->notify.addr >> PAGE_SHIFT);
-		if (pgno >= offset && pgno < offset + pages) {
-			/* No need for kmap, pages are in lowmem */
-			uint8_t *tmp = pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
-			tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
-			map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
-		}
-	}
-
-	unmap_data.unmap_ops = map->unmap_ops + offset;
-	unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
-	unmap_data.pages = map->pages + offset;
-	unmap_data.count = pages;
-
-	err = gnttab_unmap_refs_sync(&unmap_data);
-	if (err)
-		return err;
+	int i;
+	struct gntdev_grant_map *map = data->data;
+	int offset = data->unmap_ops - map->unmap_ops;
 
-	for (i = 0; i < pages; i++) {
-		if (map->unmap_ops[offset+i].status)
-			err = -EINVAL;
+	for (i = 0; i < data->count; i++) {
+		WARN_ON(map->unmap_ops[offset+i].status);
 		pr_debug("unmap handle=%d st=%d\n",
 			map->unmap_ops[offset+i].handle,
 			map->unmap_ops[offset+i].status);
 		map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
 		if (use_ptemod) {
-			if (map->kunmap_ops[offset+i].status)
-				err = -EINVAL;
+			WARN_ON(map->kunmap_ops[offset+i].status);
 			pr_debug("kunmap handle=%u st=%d\n",
 				 map->kunmap_ops[offset+i].handle,
 				 map->kunmap_ops[offset+i].status);
 			map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
 		}
 	}
-	return err;
 }
 
-static int unmap_grant_pages(struct gntdev_grant_map *map, int offset,
-			     int pages)
+static void __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
+			       int pages)
+{
+	if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
+		int pgno = (map->notify.addr >> PAGE_SHIFT);
+
+		if (pgno >= offset && pgno < offset + pages) {
+			/* No need for kmap, pages are in lowmem */
+			uint8_t *tmp = pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
+
+			tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
+			map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
+		}
+	}
+
+	map->unmap_data.unmap_ops = map->unmap_ops + offset;
+	map->unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
+	map->unmap_data.pages = map->pages + offset;
+	map->unmap_data.count = pages;
+	map->unmap_data.done = __unmap_grant_pages_done;
+	map->unmap_data.data = map;
+
+	gnttab_unmap_refs_async(&map->unmap_data);
+}
+
+static void unmap_grant_pages(struct gntdev_grant_map *map, int offset,
+			      int pages)
 {
-	int range, err = 0;
+	int range;
 
 	pr_debug("unmap %d+%d [%d+%d]\n", map->index, map->count, offset, pages);
 
 	/* It is possible the requested range will have a "hole" where we
 	 * already unmapped some of the grants. Only unmap valid ranges.
 	 */
-	while (pages && !err) {
+	while (pages) {
 		while (pages &&
 		       map->unmap_ops[offset].handle == INVALID_GRANT_HANDLE) {
 			offset++;
@@ -416,12 +420,10 @@ static int unmap_grant_pages(struct gntdev_grant_map *map, int offset,
 				break;
 			range++;
 		}
-		err = __unmap_grant_pages(map, offset, range);
+		__unmap_grant_pages(map, offset, range);
 		offset += range;
 		pages -= range;
 	}
-
-	return err;
 }
 
 /* ------------------------------------------------------------------ */
@@ -473,7 +475,6 @@ static bool gntdev_invalidate(struct mmu_interval_notifier *mn,
 	struct gntdev_grant_map *map =
 		container_of(mn, struct gntdev_grant_map, notifier);
 	unsigned long mstart, mend;
-	int err;
 
 	if (!mmu_notifier_range_blockable(range))
 		return false;
@@ -494,10 +495,9 @@ static bool gntdev_invalidate(struct mmu_interval_notifier *mn,
 			map->index, map->count,
 			map->vma->vm_start, map->vma->vm_end,
 			range->start, range->end, mstart, mend);
-	err = unmap_grant_pages(map,
+	unmap_grant_pages(map,
 				(mstart - map->vma->vm_start) >> PAGE_SHIFT,
 				(mend - mstart) >> PAGE_SHIFT);
-	WARN_ON(err);
 
 	return true;
 }
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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