Merge tag 'for-5.7-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux

Pull btrfs updates from David Sterba:
 "A number of core changes that make things work better in general, code
  is simpler and cleaner.

  Core changes:

   - per-inode file extent tree, for in memory tracking of contiguous
     extent ranges to make sure i_size adjustments are accurate

   - tree root structures are protected by reference counts, replacing
     SRCU that did not cover some cases

   - leak detector for tree root structures

   - per-transaction pinned extent tracking

   - buffer heads are replaced by bios for super block access

   - speedup of extent back reference resolution, on an example test
     scenario the runtime of send went down from a hour to minutes

   - factor out locking scheme used for subvolume writer and NOCOW
     exclusion, abstracted as DREW lock, double reader-writer exclusion
     (allow either readers or writers)

   - cleanup and abstract extent allocation policies, preparation for
     zoned device support

   - make reflink/clone_range work on inline extents

   - add more cancellation point for relocation, improves long response
     from 'balance cancel'

   - add page migration callback for data pages

   - switch to guid for uuids, with additional cleanups of the interface

   - make ranged full fsyncs more efficient

   - removal of obsolete ioctl flag BTRFS_SUBVOL_CREATE_ASYNC

   - remove b-tree readahead from delayed refs paths, avoiding seek and
     read unnecessary blocks

  Features:

   - v2 of ioctl to delete subvolumes, allowing to delete by id and more
     future extensions

  Fixes:

   - fix qgroup rescan worker that could block umount

   - fix crash during unmount due to race with delayed inode workers

   - fix dellaloc flushing logic that could create unnecessary chunks
     under heavy load

   - fix missing file extent item for hole after ranged fsync

   - several fixes in relocation error handling

  Other:

   - more documentation of relocation, device replace, space
     reservations

   - many random cleanups"

* tag 'for-5.7-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux: (210 commits)
  btrfs: fix missing semaphore unlock in btrfs_sync_file
  btrfs: use nofs allocations for running delayed items
  btrfs: sysfs: Use scnprintf() instead of snprintf()
  btrfs: do not resolve backrefs for roots that are being deleted
  btrfs: track reloc roots based on their commit root bytenr
  btrfs: restart relocate_tree_blocks properly
  btrfs: reloc: reorder reservation before root selection
  btrfs: do not readahead in build_backref_tree
  btrfs: do not use readahead for running delayed refs
  btrfs: Remove async_transid from btrfs_mksubvol/create_subvol/create_snapshot
  btrfs: Remove transid argument from btrfs_ioctl_snap_create_transid
  btrfs: Remove BTRFS_SUBVOL_CREATE_ASYNC support
  btrfs: kill the subvol_srcu
  btrfs: make btrfs_cleanup_fs_roots use the radix tree lock
  btrfs: don't take an extra root ref at allocation time
  btrfs: hold a ref on the root on the dead roots list
  btrfs: make inodes hold a ref on their roots
  btrfs: move the root freeing stuff into btrfs_put_root
  btrfs: move ino_cache_inode dropping out of btrfs_free_fs_root
  btrfs: make the extent buffer leak check per fs info
  ...

1 files changed, 42 insertions, 2 deletions

diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
index 2ca2a09d0e23..db93909b25e0 100644
--- a/fs/btrfs/dev-replace.c
+++ b/fs/btrfs/dev-replace.c
@@ -22,6 +22,46 @@
 #include "dev-replace.h"
 #include "sysfs.h"
 
+/*
+ * Device replace overview
+ *
+ * [Objective]
+ * To copy all extents (both new and on-disk) from source device to target
+ * device, while still keeping the filesystem read-write.
+ *
+ * [Method]
+ * There are two main methods involved:
+ *
+ * - Write duplication
+ *
+ *   All new writes will be written to both target and source devices, so even
+ *   if replace gets canceled, sources device still contans up-to-date data.
+ *
+ *   Location:		handle_ops_on_dev_replace() from __btrfs_map_block()
+ *   Start:		btrfs_dev_replace_start()
+ *   End:		btrfs_dev_replace_finishing()
+ *   Content:		Latest data/metadata
+ *
+ * - Copy existing extents
+ *
+ *   This happens by re-using scrub facility, as scrub also iterates through
+ *   existing extents from commit root.
+ *
+ *   Location:		scrub_write_block_to_dev_replace() from
+ *   			scrub_block_complete()
+ *   Content:		Data/meta from commit root.
+ *
+ * Due to the content difference, we need to avoid nocow write when dev-replace
+ * is happening.  This is done by marking the block group read-only and waiting
+ * for NOCOW writes.
+ *
+ * After replace is done, the finishing part is done by swapping the target and
+ * source devices.
+ *
+ *   Location:		btrfs_dev_replace_update_device_in_mapping_tree() from
+ *   			btrfs_dev_replace_finishing()
+ */
+
 static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 				       int scrub_ret);
 static void btrfs_dev_replace_update_device_in_mapping_tree(
@@ -472,7 +512,7 @@ static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
 	atomic64_set(&dev_replace->num_uncorrectable_read_errors, 0);
 	up_write(&dev_replace->rwsem);
 
-	ret = btrfs_sysfs_add_device_link(tgt_device->fs_devices, tgt_device);
+	ret = btrfs_sysfs_add_devices_dir(tgt_device->fs_devices, tgt_device);
 	if (ret)
 		btrfs_err(fs_info, "kobj add dev failed %d", ret);
 
@@ -703,7 +743,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 	mutex_unlock(&fs_info->fs_devices->device_list_mutex);
 
 	/* replace the sysfs entry */
-	btrfs_sysfs_rm_device_link(fs_info->fs_devices, src_device);
+	btrfs_sysfs_remove_devices_dir(fs_info->fs_devices, src_device);
 	btrfs_sysfs_update_devid(tgt_device);
 	btrfs_rm_dev_replace_free_srcdev(src_device);