[PATCH] bcache: limit create flash device size

[mingzhe.zou@xxxxxxxxxxxx]

From: mingzhe <mingzhe.zou@xxxxxxxxxxxx>

Currently, size is specified and not checked when creating a flash device.
This will cause a problem, IO maybe hang when creating a flash device with
the actual size of the device.

```
	if (attr == &sysfs_flash_vol_create) {
		int r;
		uint64_t v;

		strtoi_h_or_return(buf, v);

		r = bch_flash_dev_create(c, v);
		if (r)
			return r;
	}
```

Because the flash device needs some space for superblock, journal and btree.
If the size of data reaches the available size, the new IO cannot allocate
space and will hang. At this time, the gc thread will be started frequently.

Even more unreasonable, we can create flash devices larger than actual size.

```
[root@zou ~]# echo 2G > /sys/block/vdb/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdb
NAME       MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vdb        252:16   0   1G  0 disk
└─bcache0  251:0    0   2G  0 disk
```

This patch will limit the size of flash device, reserving at least 5% of
available size for the btree.

```
[root@zou ~]# echo 2G > /sys/block/vdb/bcache/set/flash_vol_create
[root@zou ~]# lsblk /dev/vdb
NAME       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdb        252:16   0    1G  0 disk
└─bcache0  251:0    0  950M  0 disk
```

Signed-off-by: mingzhe <mingzhe.zou@xxxxxxxxxxxx>
---
 drivers/md/bcache/super.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index ba3909bb6bea..f41e09e0e8ee 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1579,6 +1579,17 @@ static int flash_devs_run(struct cache_set *c)
 	return ret;
 }
 
+static inline sector_t flash_dev_max_sectors(struct cache_set *c)
+{
+	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 * 95);
+}
+
 int bch_flash_dev_create(struct cache_set *c, uint64_t size)
 {
 	struct uuid_entry *u;
@@ -1600,7 +1611,7 @@ int bch_flash_dev_create(struct cache_set *c, uint64_t size)
 	u->first_reg = u->last_reg = cpu_to_le32((u32)ktime_get_real_seconds());
 
 	SET_UUID_FLASH_ONLY(u, 1);
-	u->sectors = size >> 9;
+	u->sectors = min(flash_dev_max_sectors(c), size >> 9);
 
 	bch_uuid_write(c);
 
-- 
2.17.1