null_blk: Introduce fua attribute

Add the fua configfs attribute and module parameter to allow
configuring if the device supports FUA or not. Using this attribute
has an effect on the null_blk device only if memory backing is enabled
together with a write cache (cache_size option).

This new attribute allows configuring a null_blk device with a write
cache but without FUA support. This is convenient to test the block
layer flush machinery.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Tested-by: Hans Holmberg <hans.holmberg@wdc.com>
Tested-by: Dennis Maisenbacher <dennis.maisenbacher@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20240408014128.205141-18-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: damien-lemoal

drivers/block/null_blk/main.c

@@ -225,6 +225,10 @@ static unsigned long g_cache_size;
 module_param_named(cache_size, g_cache_size, ulong, 0444);
 MODULE_PARM_DESC(mbps, "Cache size in MiB for memory-backed device. Default: 0 (none)");
 
+static bool g_fua = true;
+module_param_named(fua, g_fua, bool, 0444);
+MODULE_PARM_DESC(zoned, "Enable/disable FUA support when cache_size is used. Default: true");
+
 static unsigned int g_mbps;
 module_param_named(mbps, g_mbps, uint, 0444);
 MODULE_PARM_DESC(mbps, "Limit maximum bandwidth (in MiB/s). Default: 0 (no limit)");
@@ -446,6 +450,7 @@ NULLB_DEVICE_ATTR(virt_boundary, bool, NULL);
 NULLB_DEVICE_ATTR(no_sched, bool, NULL);
 NULLB_DEVICE_ATTR(shared_tags, bool, NULL);
 NULLB_DEVICE_ATTR(shared_tag_bitmap, bool, NULL);
+NULLB_DEVICE_ATTR(fua, bool, NULL);
 
 static ssize_t nullb_device_power_show(struct config_item *item, char *page)
 {
@@ -593,6 +598,7 @@ static struct configfs_attribute *nullb_device_attrs[] = {
 	&nullb_device_attr_no_sched,
 	&nullb_device_attr_shared_tags,
 	&nullb_device_attr_shared_tag_bitmap,
+	&nullb_device_attr_fua,
 	NULL,
 };
 
@@ -671,7 +677,7 @@ nullb_group_drop_item(struct config_group *group, struct config_item *item)
 static ssize_t memb_group_features_show(struct config_item *item, char *page)
 {
 	return snprintf(page, PAGE_SIZE,
-			"badblocks,blocking,blocksize,cache_size,"
+			"badblocks,blocking,blocksize,cache_size,fua,"
 			"completion_nsec,discard,home_node,hw_queue_depth,"
 			"irqmode,max_sectors,mbps,memory_backed,no_sched,"
 			"poll_queues,power,queue_mode,shared_tag_bitmap,"
@@ -763,6 +769,8 @@ static struct nullb_device *null_alloc_dev(void)
 	dev->no_sched = g_no_sched;
 	dev->shared_tags = g_shared_tags;
 	dev->shared_tag_bitmap = g_shared_tag_bitmap;
+	dev->fua = g_fua;
+
 	return dev;
 }
 
@@ -1920,7 +1928,7 @@ static int null_add_dev(struct nullb_device *dev)
 
 	if (dev->cache_size > 0) {
 		set_bit(NULLB_DEV_FL_CACHE, &nullb->dev->flags);
-		blk_queue_write_cache(nullb->q, true, true);
+		blk_queue_write_cache(nullb->q, true, dev->fua);
 	}
 
 	nullb->q->queuedata = nullb;

drivers/block/null_blk/null_blk.h

@@ -105,6 +105,7 @@ struct nullb_device {
 	bool no_sched; /* no IO scheduler for the device */
 	bool shared_tags; /* share tag set between devices for blk-mq */
 	bool shared_tag_bitmap; /* use hostwide shared tags */
+	bool fua; /* Support FUA */
 };
 
 struct nullb {