linux-kernel - Re: [PATCH] btrfs: let btrfs_delete_unused

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Date:   Mon, 10 Oct 2016 17:04:03 -0400
From:   Chris Mason <clm@...com>
To:     Naohiro Aota <naohiro.aota@...t.com>, <linux-btrfs@...r.kernel.org>
CC:     Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] btrfs: let btrfs_delete_unused_bgs() to clean relocated
 bgs

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@...t.com>

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.

-chris