[PATCH v2] bcache: limit multiple flash devices size

[mingzhe.zou@easystack.cn]

From: mingzhe <mingzhe.zou@easystack.cn>

Bcache allows multiple flash devices to be created on the same cache.
We can create multiple flash devices, and the total size larger than
cache device's actual size.
```
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
└─bcache1  251:128  0   50G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
├─bcache2  251:256  0   50G  0 disk
└─bcache1  251:128  0   50G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
├─bcache3  251:256  0   50G  0 disk
├─bcache2  251:256  0   50G  0 disk
└─bcache1  251:128  0   50G  0 disk
```

This patch will limit the total size of multi-flash device, until no
free space to create a new flash device with an error.
```
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
└─bcache1  251:128  0   50G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
├─bcache2  251:256  0 39.9G  0 disk
└─bcache1  251:128  0   50G  0 disk
[root@zou ~]# echo 50G > /sys/block/vdd/bcache/set/flash_vol_create
-bash: echo: write error: No space left on device
[root@zou ~]# lsblk /dev/vdd
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdd        252:48   0  100G  0 disk
├─bcache2  251:256  0 39.9G  0 disk
└─bcache1  251:128  0   50G  0 disk
```

Signed-off-by: mingzhe <mingzhe.zou@easystack.cn>
---
 drivers/md/bcache/super.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 214a384dc1d7..d4b3ab078ad3 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1581,13 +1581,32 @@ static int flash_devs_run(struct cache_set *c)
 
 static inline sector_t flash_dev_max_sectors(struct cache_set *c)
 {
+	sector_t sectors;
+	struct uuid_entry *u;
+	struct bcache_device *d;
 	size_t avail_nbuckets;
 	struct cache *ca = c->cache;
 	size_t first_bucket = ca->sb.first_bucket;
 	size_t njournal_buckets = ca->sb.njournal_buckets;
 
 	avail_nbuckets = c->nbuckets - first_bucket - njournal_buckets;
-	return bucket_to_sector(c, avail_nbuckets / 100 * FLASH_DEV_AVAILABLE_RATIO);
+	sectors = bucket_to_sector(c, avail_nbuckets / 100 * FLASH_DEV_AVAILABLE_RATIO);
+
+	/*
+	 * Although 10% of buckets are forced to reserve, it should
+	 * be able to meet the needs of gc and btree node allocation.
+	 * But the remaining buckets are not necessarily allocatable
+	 * free buckets. Because some of the buckets might be cached
+	 * dirty data. At the same time, the space that has been
+	 * allocated to other flash devices must be considered.
+	 */
+	for (u = c->uuids; u < c->uuids + c->nr_uuids && sectors > 0; u++) {
+		if (!(d = c->devices[u - c->uuids]))
+			continue;
+		sectors -= min(UUID_FLASH_ONLY(u) ? u->sectors :
+			       bcache_dev_sectors_dirty(d), sectors);
+	}
+	return sectors;
 }
 
 int bch_flash_dev_create(struct cache_set *c, uint64_t size)
@@ -1612,6 +1631,10 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)
 
 	SET_UUID_FLASH_ONLY(u, 1);
 	u->sectors = min(flash_dev_max_sectors(c), size >> 9);
+	if (!u->sectors) {
+		pr_err("Can't create volume, no free space");
+		return -ENOSPC;
+	}
 
 	bch_uuid_write(c);
 
-- 
2.17.1