[PATCH] dm table: propagate BDI_CAP_STABLE_WRITES

[Ilya Dryomov]

Some devices don't use blk_integrity but still want stable pages
because they do their own checksumming.  Examples include rbd and iSCSI
when data digests are negotiated.  Stacking DM (and thus LVM) on top of
these devices results in sporadic checksum errors.

Set BDI_CAP_STABLE_WRITES if any underlying device has it set.

Signed-off-by: Ilya Dryomov <idryomov@xxxxxxxxx>
---
 drivers/md/dm-table.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index ba9481f1bf3c..cde3b49b2a91 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1844,6 +1844,36 @@ static bool dm_table_supports_secure_erase(struct dm_table *t)
 	return true;
 }
 
+static int device_requires_stable_pages(struct dm_target *ti,
+					struct dm_dev *dev, sector_t start,
+					sector_t len, void *data)
+{
+	struct request_queue *q = bdev_get_queue(dev->bdev);
+
+	return q && bdi_cap_stable_pages_required(q->backing_dev_info);
+}
+
+/*
+ * If any underlying device requires stable pages, a table must require
+ * them as well.  Only targets that support iterate_devices are considered:
+ * don't want error, zero, etc to require stable pages.
+ */
+static bool dm_table_requires_stable_pages(struct dm_table *t)
+{
+	struct dm_target *ti;
+	unsigned i;
+
+	for (i = 0; i < dm_table_get_num_targets(t); i++) {
+		ti = dm_table_get_target(t, i);
+
+		if (ti->type->iterate_devices &&
+		    ti->type->iterate_devices(ti, device_requires_stable_pages, NULL))
+			return true;
+	}
+
+	return false;
+}
+
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 			       struct queue_limits *limits)
 {
@@ -1896,6 +1926,15 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
 
 	dm_table_verify_integrity(t);
 
+	/*
+	 * Some devices don't use blk_integrity but still want stable pages
+	 * because they do their own checksumming.
+	 */
+	if (dm_table_requires_stable_pages(t))
+		q->backing_dev_info->capabilities |= BDI_CAP_STABLE_WRITES;
+	else
+		q->backing_dev_info->capabilities &= ~BDI_CAP_STABLE_WRITES;
+
 	/*
 	 * Determine whether or not this queue's I/O timings contribute
 	 * to the entropy pool, Only request-based targets use this.
-- 
2.19.2