btrfs-progs: check: add the ability to reset btrfs_dev_item::bytes_used

There is a report from the mailing list that one user got its filesystem
with device item bytes_used mismatch.

This problem leaves the device item with some ghost bytes_used, meaning
even if we delete all device extents of that device, the bytes_used
still won't be 0.

This itself is not a big deal, but when the user used up all its
unallocated space, write time tree-checker can be triggered and make the
fs RO, as the new device::bytes_used can be larger than
device::total_bytes.

Thus we need to fix the problem in btrfs-check to avoid above write-time
tree check warning.

This patch will add the ability to reset a device's bytes_used to both
original mode and lowmem mode.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

check/main.c

@@ -8397,11 +8397,17 @@ static int check_device_used(struct device_record *dev_rec,
 	}
 
 	if (total_byte != dev_rec->byte_used) {
+		int ret = -1;
+
 		fprintf(stderr,
 			"Dev extent's total-byte(%llu) is not equal to byte-used(%llu) in dev[%llu, %u, %llu]\n",
 			total_byte, dev_rec->byte_used,	dev_rec->objectid,
 			dev_rec->type, dev_rec->offset);
-		return -1;
+		if (repair) {
+			ret = repair_dev_item_bytes_used(gfs_info,
+					dev_rec->devid, total_byte);
+		}
+		return ret;
 	} else {
 		return 0;
 	}

check/mode-common.c

@@ -21,6 +21,7 @@
 #include "kernel-shared/transaction.h"
 #include "common/utils.h"
 #include "kernel-shared/disk-io.h"
+#include "kernel-shared/volumes.h"
 #include "common/repair.h"
 #include "check/mode-common.h"
 
@@ -1243,3 +1244,60 @@ out:
 	btrfs_release_path(&path);
 	return ret;
 }
+
+int repair_dev_item_bytes_used(struct btrfs_fs_info *fs_info,
+			       u64 devid, u64 bytes_used_expected)
+{
+	struct btrfs_trans_handle *trans;
+	struct btrfs_device *device;
+	int ret;
+
+	device = btrfs_find_device_by_devid(fs_info->fs_devices, devid, 0);
+	if (!device) {
+		error("failed to find device with devid %llu", devid);
+		return -ENOENT;
+	}
+
+	/* Bytes_used matches, not what we can repair */
+	if (device->bytes_used == bytes_used_expected)
+		return -ENOTSUP;
+
+	/*
+	 * We have to set the device bytes_used right now, before starting a
+	 * new transaction, as it may allocate a new chunk and modify
+	 * device->bytes_used.
+	 */
+	device->bytes_used = bytes_used_expected;
+	trans = btrfs_start_transaction(fs_info->chunk_root, 1);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		errno = -ret;
+		error("failed to start transaction: %m");
+		return ret;
+	}
+
+	/* Manually update the device item in chunk tree */
+	ret = btrfs_update_device(trans, device);
+	if (ret < 0) {
+		errno = -ret;
+		error("failed to update device item for devid %llu: %m", devid);
+		goto error;
+	}
+
+	/*
+	 * Commit transaction not only to save the above change but also update
+	 * the device item in super block.
+	 */
+	ret = btrfs_commit_transaction(trans, fs_info->chunk_root);
+	if (ret < 0) {
+		errno = -ret;
+		error("failed to commit transaction: %m");
+	} else {
+		printf("reset devid %llu bytes_used to %llu\n", devid,
+		       device->bytes_used);
+	}
+	return ret;
+error:
+	btrfs_abort_transaction(trans, ret);
+	return ret;
+}

check/mode-common.h

@@ -192,4 +192,7 @@ static inline void btrfs_check_subpage_eb_alignment(u64 start, u32 len)
 			start, start + len);
 }
 
+int repair_dev_item_bytes_used(struct btrfs_fs_info *fs_info,
+			       u64 devid, u64 bytes_used_expected);
+
 #endif

check/mode-lowmem.c

@@ -4454,7 +4454,8 @@ out:
 /*
  * Check if the used space is correct with the dev item
  */
-static int check_dev_item(struct extent_buffer *eb, int slot)
+static int check_dev_item(struct extent_buffer *eb, int slot,
+			  u64 *bytes_used_expected)
 {
 	struct btrfs_root *dev_root = gfs_info->dev_root;
 	struct btrfs_dev_item *dev_item;
@@ -4543,6 +4544,7 @@ next:
 	}
 	btrfs_release_path(&path);
 
+	*bytes_used_expected = total;
 	if (used != total) {
 		btrfs_item_key_to_cpu(eb, &key, slot);
 		error(
@@ -4744,6 +4746,7 @@ static int repair_chunk_item(struct btrfs_root *chunk_root,
 static int check_leaf_items(struct btrfs_root *root, struct btrfs_path *path,
 			    struct node_refs *nrefs, int account_bytes)
 {
+	u64 bytes_used_expected = (u64)-1;
 	struct btrfs_key key;
 	struct extent_buffer *eb;
 	int slot;
@@ -4782,7 +4785,14 @@ again:
 		err |= ret;
 		break;
 	case BTRFS_DEV_ITEM_KEY:
-		ret = check_dev_item(eb, slot);
+		ret = check_dev_item(eb, slot, &bytes_used_expected);
+		if (repair && (ret & ACCOUNTING_MISMATCH) &&
+		    bytes_used_expected != (u64)-1) {
+			ret = repair_dev_item_bytes_used(root->fs_info,
+					key.offset, bytes_used_expected);
+			if (ret < 0)
+				ret = ACCOUNTING_MISMATCH;
+		}
 		err |= ret;
 		break;
 	case BTRFS_CHUNK_ITEM_KEY: