Re: [PATCH 1/5] btrfs: check rw_devices, not num_devices for restriping

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/1/11 上午12:11, Josef Bacik wrote:
> While running xfstests with compression on I noticed I was panicing on
> btrfs/154.  I bisected this down to my inc_block_group_ro patches, which
> was strange.
> 
> What was happening is with my patches we now use btrfs_can_overcommit()
> to see if we can flip a block group read only.  Before this would fail
> because we weren't taking into account the usable un-allocated space for
> allocating chunks.  With my patches we were allowed to do the balance,
> which is technically correct.
> 
> However this test is testing restriping with a degraded mount, something
> that isn't working right because Anand's fix for the test was never
> actually merged.
> 
> So now we're trying to allocate a chunk and cannot because we want to
> allocate a RAID1 chunk, but there's only 1 device that's available for
> usage.  This results in an ENOSPC in one of the BUG_ON(ret) paths in
> relocation (and a tricky path that is going to take many more patches to
> fix.)
> 
> But we shouldn't even be making it this far, we don't have enough
> devices to restripe.  The problem is we're using btrfs_num_devices(),
> which for some reason includes missing devices.  That's not actually
> what we want, we want the rw_devices.
> 
> Fix this by getting the rw_devices.  With this patch we're no longer
> panicing with my other patches applied, and we're in fact erroring out
> at the correct spot instead of at inc_block_group_ro.  The fact that
> this was working before was just sheer dumb luck.
> 
> Fixes: e4d8ec0f65b9 ("Btrfs: implement online profile changing")
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/volumes.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 7483521a928b..a92059555754 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3881,7 +3881,14 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>  		}
>  	}
>  
> -	num_devices = btrfs_num_devices(fs_info);
> +	/*
> +	 * rw_devices can be messed with by rm_device and device replace, so
> +	 * take the chunk_mutex to make sure we have a relatively consistent
> +	 * view of the fs at this point.
> +	 */
> +	mutex_lock(&fs_info->chunk_mutex);
> +	num_devices = fs_info->fs_devices->rw_devices;
> +	mutex_unlock(&fs_info->chunk_mutex);

chunk_mutex is the correct lock for rw_devices counter and alloc_list.
So,

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

>  
>  	/*
>  	 * SINGLE profile on-disk has no profile bit, but in-memory we have a
> 


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