Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated bgs

From: Chris Mason
Date: Mon Oct 10 2016 - 17:16:33 EST

On 09/02/2016 03:46 AM, Naohiro Aota wrote:
Currently, btrfs_relocate_chunk() is removing relocated BG by itself. But
the work can be done by btrfs_delete_unused_bgs() (and it's better since it
trim the BG). Let's dedupe the code.

While btrfs_delete_unused_bgs() is already hitting the relocated BG, it
skip the BG since the BG has "ro" flag set (to keep balancing BG intact).
On the other hand, btrfs cannot drop "ro" flag here to prevent additional
writes. So this patch make use of "removed" flag.
btrfs_delete_unused_bgs() now detect the flag to distinguish whether a
read-only BG is relocating or not.

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

This runs into trouble with btrfs_rm_device(), I've been triggering crashes with btrfs/101 here.

The problem is that by the time we get around to running btrfs_delete_unused_bgs(), btrfs_rm_device() has long since free'd the device.

I thought about calling btrfs_delete_unused_bgs() directly from btrfs_rm_device(), but it might bail out without returning an error for a number of reasons.

For now, I've reverted this patch from the pull, but we can bring it back once the device removal path is covered.