[PATCH v2 5/5] btrfs: don't trim the target of a device replace

[Josef Bacik <josef@toxicpanda.com>]

FITRIM also discards the space of every device that is not allocated to a
chunk. btrfs_trim_free_extents() goes over fs_devices->devices and, for
each device that is writeable, discards the ranges that have neither
CHUNK_ALLOCATED nor CHUNK_TRIMMED set in device->alloc_state.

The target device of a running replace operation is on that list and it is
writeable. But its alloc_state is empty. The allocated ranges of the
source device are only copied to it by btrfs_set_target_alloc_state(),
when the replace finishes. Until then the whole device looks unallocated,
so an fstrim discards everything that was copied to it so far, and what is
being written to it in parallel to the source device.

The replace finishes without any error and the source device is dropped.
With a single device filesystem, 700M of data and fstrim called in a loop
while the replace runs:

  BTRFS info: dev_replace from /dev/vdb (devid 1) to /dev/vdc finished
  BTRFS error: bad tree block start, mirror 1 want 30638080 have 0
  BTRFS error: bad tree block start, mirror 2 want 30638080 have 0
  BTRFS info: scrub: not finished on devid 1 with status: -5

Skip the target device. The flag is cleared with the device list mutex
held, after the alloc_state was set up, and the trimming code holds that
mutex while it works on a device.

Fixes: 499f377f49f0 ("btrfs: iterate over unused chunk space in FITRIM")
CC: stable@vger.kernel.org
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/extent-tree.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 9b0a5c6b96f3..22155507f905 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -6641,6 +6641,14 @@ static int btrfs_trim_free_extents_throttle(struct btrfs_device *device,
 	if (!test_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state) || !device->bdev)
 		return 0;
 
+	/*
+	 * The target of a running device replace has nothing in its alloc_state
+	 * until the replace finishes, all of it would look unallocated and we
+	 * would discard what was already copied to it.
+	 */
+	if (test_bit(BTRFS_DEV_STATE_REPLACE_TGT, &device->dev_state))
+		return 0;
+
 	/* Discard not supported = nothing to do. */
 	if (!bdev_max_discard_sectors(device->bdev))
 		return 0;

-- 
2.55.0