[PATCH v5 3/8] nbd: reset write cache on disconnect

An inactive nbd device may refuse any I/O operations. The nbd_config_put
function calls invalidate_disk, which sets the device capacity to zero
to reject all read and write I/O. For zero-sector flush I/O requests
from blkdev_issue_flush, if the write cache is disabled, the zero-sector
flush I/O immediately returns 0 in submit_bio_noacct. However, since
nbd_config_put does not clear the write cache state, an inactive nbd
device might still have the write cache enabled. In this situation,
zero-sector flush I/O will return -EIO because there is no active socket.
Although this does not cause any bugs, resetting the write cache state
would make the behavior consistent.

Signed-off-by: Yang Erkun <[email protected]>
---
 drivers/block/nbd.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 7ec85f94f742..da4b48ef3c79 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1469,8 +1469,13 @@ static void nbd_config_put(struct nbd_device *nbd)
 	if (refcount_dec_and_mutex_lock(&nbd->config_refs,
 					&nbd->config_lock)) {
 		struct nbd_config *config = nbd->config;
+		struct queue_limits lim;
 		nbd_dev_dbg_close(nbd);
 		invalidate_disk(nbd->disk);
+		/* reset write cache */
+		lim = queue_limits_start_update(nbd->disk->queue);
+		lim.features &= ~(BLK_FEAT_WRITE_CACHE | BLK_FEAT_FUA);
+		queue_limits_commit_update(nbd->disk->queue, &lim);
 		if (nbd->config->bytesize)
 			kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
 		if (test_and_clear_bit(NBD_RT_HAS_PID_FILE,
-- 
2.52.0