libblkid: add magic and probing for zoned btrfs

This commit adds zone-aware magics and probing functions for zoned btrfs.

The superblock (and its copies) are the only data structure in btrfs with a
fixed location on a device. Since we cannot do overwrites in a sequential
write required zone, we cannot place the superblock in the zone.

Thus, zoned btrfs uses superblock log writing to update superblocks on
sequential write required zones. It uses two zones as a circular buffer to
write updated superblocks. Once the first zone is filled up, start writing
into the second buffer. When both zones are filled up, and before starting
to write to the first zone again, it reset the first zone.

We can determine the position of the latest superblock by reading the write
pointer information from a device. One corner case is when both zones are
full. For this situation, we read out the last superblock of each zone and
compare them to determine which zone is older.

The magics can detect a superblock magic ("_BHRfs_M") at the beginning of
zone #0 or zone #1 to see if it is zoned btrfs. When both zones are filled
up, zoned btrfs resets the first zone to write a new superblock. If btrfs
crashes at the moment, we do not see a superblock at zone #0. Thus, we need
to check not only zone #0 but also zone #1.

It also supports the temporary magic ("!BHRfS_M") in zone #0. Mkfs.btrfs
first writes the temporary superblock to the zone during the mkfs process.
It will survive there until the zones are filled up and reset. So, we also
need to detect this temporary magic.

Finally, this commit extends probe_btrfs() to load the latest superblock
determined by the write pointers.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

include/blkdev.h

@@ -15,6 +15,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <stdint.h>
 
 #ifdef HAVE_SYS_MKDEV_H
 # include <sys/mkdev.h>		/* major and minor on Solaris */
@@ -147,5 +148,13 @@ int blkdev_get_geometry(int fd, unsigned int *h, unsigned int *s);
 const char *blkdev_scsi_type_to_name(int type);
 
 int blkdev_lock(int fd, const char *devname, const char *lockmode);
+#ifdef HAVE_LINUX_BLKZONED_H
+struct blk_zone_report *blkdev_get_zonereport(int fd, uint64_t sector, uint32_t nzones);
+#else
+static inline struct blk_zone_report *blkdev_get_zonereport(int fd, uint64_t sector, uint32_t nzones)
+{
+	return NULL;
+}
+#endif
 
 #endif /* BLKDEV_H */

lib/blkdev.c

@@ -15,6 +15,10 @@
 #include <linux/fd.h>
 #endif
 
+#ifdef HAVE_LINUX_BLKZONED_H
+#include <linux/blkzoned.h>
+#endif
+
 #ifdef HAVE_SYS_DISKLABEL_H
 #include <sys/disklabel.h>
 #endif
@@ -412,6 +416,31 @@ int blkdev_lock(int fd, const char *devname, const char *lockmode)
 	return rc;
 }
 
+#ifdef HAVE_LINUX_BLKZONED_H
+struct blk_zone_report *blkdev_get_zonereport(int fd, uint64_t sector, uint32_t nzones)
+{
+	struct blk_zone_report *rep;
+	size_t rep_size;
+	int ret;
+
+	rep_size = sizeof(struct blk_zone_report) + sizeof(struct blk_zone) * 2;
+	rep = calloc(1, rep_size);
+	if (!rep)
+		return NULL;
+
+	rep->sector = sector;
+	rep->nr_zones = nzones;
+
+	ret = ioctl(fd, BLKREPORTZONE, rep);
+	if (ret || rep->nr_zones != nzones) {
+		free(rep);
+		return NULL;
+	}
+
+	return rep;
+}
+#endif
+
 
 #ifdef TEST_PROGRAM_BLKDEV
 #include <stdio.h>

libblkid/src/superblocks/btrfs.c

@@ -9,6 +9,12 @@
 #include <unistd.h>
 #include <string.h>
 #include <stdint.h>
+#include <stdbool.h>
+#include <assert.h>
+
+#ifdef HAVE_LINUX_BLKZONED_H
+#include <linux/blkzoned.h>
+#endif
 
 #include "superblocks.h"
 
@@ -59,11 +65,157 @@ struct btrfs_super_block {
 	uint8_t label[256];
 } __attribute__ ((__packed__));
 
+#define BTRFS_SUPER_INFO_SIZE 4096
+
+/* Number of superblock log zones */
+#define BTRFS_NR_SB_LOG_ZONES 2
+
+/* Introduce some macros and types to unify the code with kernel side */
+#define SECTOR_SHIFT 9
+
+typedef uint64_t sector_t;
+
+#ifdef HAVE_LINUX_BLKZONED_H
+static int sb_write_pointer(blkid_probe pr, struct blk_zone *zones, uint64_t *wp_ret)
+{
+	bool empty[BTRFS_NR_SB_LOG_ZONES];
+	bool full[BTRFS_NR_SB_LOG_ZONES];
+	sector_t sector;
+
+	assert(zones[0].type != BLK_ZONE_TYPE_CONVENTIONAL &&
+	       zones[1].type != BLK_ZONE_TYPE_CONVENTIONAL);
+
+	empty[0] = zones[0].cond == BLK_ZONE_COND_EMPTY;
+	empty[1] = zones[1].cond == BLK_ZONE_COND_EMPTY;
+	full[0] = zones[0].cond == BLK_ZONE_COND_FULL;
+	full[1] = zones[1].cond == BLK_ZONE_COND_FULL;
+
+	/*
+	 * Possible states of log buffer zones
+	 *
+	 *           Empty[0]  In use[0]  Full[0]
+	 * Empty[1]         *          x        0
+	 * In use[1]        0          x        0
+	 * Full[1]          1          1        C
+	 *
+	 * Log position:
+	 *   *: Special case, no superblock is written
+	 *   0: Use write pointer of zones[0]
+	 *   1: Use write pointer of zones[1]
+	 *   C: Compare super blcoks from zones[0] and zones[1], use the latest
+	 *      one determined by generation
+	 *   x: Invalid state
+	 */
+
+	if (empty[0] && empty[1]) {
+		/* Special case to distinguish no superblock to read */
+		*wp_ret = zones[0].start << SECTOR_SHIFT;
+		return -ENOENT;
+	} else if (full[0] && full[1]) {
+		/* Compare two super blocks */
+		struct btrfs_super_block *super[BTRFS_NR_SB_LOG_ZONES];
+		int i;
+
+		for (i = 0; i < BTRFS_NR_SB_LOG_ZONES; i++) {
+			uint64_t bytenr;
+
+			bytenr = ((zones[i].start + zones[i].len)
+				   << SECTOR_SHIFT) - BTRFS_SUPER_INFO_SIZE;
+
+			super[i] = (struct btrfs_super_block *)
+				blkid_probe_get_buffer(pr, bytenr, BTRFS_SUPER_INFO_SIZE);
+			if (!super[i])
+				return -EIO;
+		}
+
+		if (super[0]->generation > super[1]->generation)
+			sector = zones[1].start;
+		else
+			sector = zones[0].start;
+	} else if (!full[0] && (empty[1] || full[1])) {
+		sector = zones[0].wp;
+	} else if (full[0]) {
+		sector = zones[1].wp;
+	} else {
+		return -EUCLEAN;
+	}
+	*wp_ret = sector << SECTOR_SHIFT;
+	return 0;
+}
+
+static int sb_log_offset(blkid_probe pr, uint64_t *bytenr_ret)
+{
+	uint32_t zone_num = 0;
+	uint32_t zone_size_sector;
+	struct blk_zone_report *rep;
+	struct blk_zone *zones;
+	int ret;
+	int i;
+	uint64_t wp;
+
+
+	zone_size_sector = pr->zone_size >> SECTOR_SHIFT;
+	rep = blkdev_get_zonereport(pr->fd, zone_num * zone_size_sector, 2);
+	if (!rep) {
+		ret = -errno;
+		goto out;
+	}
+	zones = (struct blk_zone *)(rep + 1);
+
+	/*
+	 * Use the head of the first conventional zone, if the zones
+	 * contain one.
+	 */
+	for (i = 0; i < BTRFS_NR_SB_LOG_ZONES; i++) {
+		if (zones[i].type == BLK_ZONE_TYPE_CONVENTIONAL) {
+			*bytenr_ret = zones[i].start << SECTOR_SHIFT;
+			ret = 0;
+			goto out;
+		}
+	}
+
+	ret = sb_write_pointer(pr, zones, &wp);
+	if (ret != -ENOENT && ret) {
+		ret = 1;
+		goto out;
+	}
+	if (ret != -ENOENT) {
+		if (wp == zones[0].start << SECTOR_SHIFT)
+			wp = (zones[1].start + zones[1].len) << SECTOR_SHIFT;
+		wp -= BTRFS_SUPER_INFO_SIZE;
+	}
+	*bytenr_ret = wp;
+
+	ret = 0;
+out:
+	free(rep);
+
+	return ret;
+}
+#endif
+
 static int probe_btrfs(blkid_probe pr, const struct blkid_idmag *mag)
 {
 	struct btrfs_super_block *bfs;
 
-	bfs = blkid_probe_get_sb(pr, mag, struct btrfs_super_block);
+	if (pr->zone_size) {
+#ifdef HAVE_LINUX_BLKZONED_H
+		uint64_t offset = 0;
+		int ret;
+
+		ret = sb_log_offset(pr, &offset);
+		if (ret)
+			return ret;
+		bfs = (struct btrfs_super_block *)
+			blkid_probe_get_buffer(pr, offset,
+					       sizeof(struct btrfs_super_block));
+#else
+		/* Nothing can be done */
+		return 1;
+#endif
+	} else {
+		bfs = blkid_probe_get_sb(pr, mag, struct btrfs_super_block);
+	}
 	if (!bfs)
 		return errno ? -errno : 1;
 
@@ -88,6 +240,11 @@ const struct blkid_idinfo btrfs_idinfo =
 	.magics		=
 	{
 	  { .magic = "_BHRfS_M", .len = 8, .sboff = 0x40, .kboff = 64 },
+	  /* For zoned btrfs */
+	  { .magic = "_BHRfS_M", .len = 8, .sboff = 0x40,
+	    .is_zoned = 1, .zonenum = 0, .kboff_inzone = 0 },
+	  { .magic = "_BHRfS_M", .len = 8, .sboff = 0x40,
+	    .is_zoned = 1, .zonenum = 1, .kboff_inzone = 0 },
 	  { NULL }
 	}
 };