[PATCH] btrfs: zoned: introduce a minimal zone size and reject mount

* [PATCH] btrfs: zoned: introduce a minimal zone size and reject mount
@ 2022-02-10 13:47 Johannes Thumshirn
  2022-02-10 14:32 ` David Sterba
  0 siblings, 1 reply; 3+ messages in thread
From: Johannes Thumshirn @ 2022-02-10 13:47 UTC (permalink / raw)
  To: David Sterba
  Cc: Johannes Thumshirn, Javier González, Naohiro Aota, linux-btrfs

When creating a filesystem on a zoned block device with a zone size of
32MB or 16MB successive mounting of this filesystem fails with the
following error message:
 host:/ # mount /dev/nullb0 /mnt/
  BTRFS info (device nullb0): flagging fs with big metadata feature
  BTRFS info (device nullb0): using free space tree
  BTRFS info (device nullb0): has skinny extents
  BTRFS info (device nullb0): host-managed zoned block device /dev/nullb0, 400 zones of 33554432 bytes
  BTRFS info (device nullb0): zoned mode enabled with zone size 33554432
  BTRFS error (device nullb0): zoned: block group 67108864 must not contain super block
  BTRFS error (device nullb0): failed to read block groups: -117
  BTRFS error (device nullb0): open_ctree failed
 mount: /mnt: wrong fs type, bad option, bad superblock on /dev/nullb0, missing codepage or helper program, or other error.

This happens because mkfs.btrfs places the system block-group exactly at
the location where regular btrfs would have it's 1st super block mirror.
In case of a 16MiB filesystem, mkfs.btrfs will place the 1st metadata
block-group at this location.

As the smallest zone size on the market today is 64MiB and we're expecting
zone sizes to be more in the 256MiB - 4GiB region, refuse to mount a
filesystem with a zone size of 32MiB or smaller.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
 fs/btrfs/zoned.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

@ Javier, I've CCed you because I've heared rumors that you guys have ZNS
dddrives with a zone size of 64MB. So I'd like to hear your toughts on the
patch as well, so I'm not breaking existing devices supported by Linux.

diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
index b7b5fac1c779..3cf9217f6fcf 100644
--- a/fs/btrfs/zoned.c
+++ b/fs/btrfs/zoned.c
@@ -51,11 +51,13 @@
 #define BTRFS_MIN_ACTIVE_ZONES		(BTRFS_SUPER_MIRROR_MAX + 5)
 
 /*
- * Maximum supported zone size. Currently, SMR disks have a zone size of
- * 256MiB, and we are expecting ZNS drives to be in the 1-4GiB range. We do not
- * expect the zone size to become larger than 8GiB in the near future.
+ * Minimum / maximum supported zone size. Currently, SMR disks have a zone
+ * size of 256MiB, and we are expecting ZNS drives to be in the 1-4GiB range.
+ * We do not expect the zone size to become larger than 8GiB or smaller than
+ * 64MiB in the near future.
  */
 #define BTRFS_MAX_ZONE_SIZE		SZ_8G
+#define BTRFS_MIN_ZONE_SIZE		SZ_64M
 
 #define SUPER_INFO_SECTORS	((u64)BTRFS_SUPER_INFO_SIZE >> SECTOR_SHIFT)
 
@@ -402,6 +404,13 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
 				 zone_info->zone_size, BTRFS_MAX_ZONE_SIZE);
 		ret = -EINVAL;
 		goto out;
+	} else if (zone_info->zone_size < BTRFS_MIN_ZONE_SIZE) {
+		btrfs_err_in_rcu(fs_info,
+		"zoned: %s: zone size %llu smaller than supported minimum %u",
+				 rcu_str_deref(device->name),
+				 zone_info->zone_size, BTRFS_MIN_ZONE_SIZE);
+		ret = -EINVAL;
+		goto out;
 	}
 
 	nr_sectors = bdev_nr_sectors(bdev);
-- 
2.34.1


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