On 9.01.20 г. 0:39 ч., 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@xxxxxxxxxxxxxx>
> ---
> fs/btrfs/volumes.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 7483521a928b..ee4d440e544e 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -3881,7 +3881,13 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
> }
> }
>
> - num_devices = btrfs_num_devices(fs_info);
> + /*
> + * device replace only adjusts rw_devices when it is finishing, so take
> + * the lock here to make sure we get the right value for rw_devices.
> + */
> + down_read(&fs_info->dev_replace.rwsem);
> + num_devices = fs_info->fs_devices->rw_devices;
> + up_read(&fs_info->dev_replace.rwsem);
rw_devices is modified under a myriad of locks, to name a few:
under chunk/device_list mutex in init_new_device
under device_lsit_mutex in
btrfs_open_devices->open_fs_devices->btrfs_open_one_device
uuid mutex in btrfs_rm_device and also under chunk_mutex in an error
path in the same function.
device_list_mutex in btrfs_rm_dev_replace_remove_srcdev
SO you are only protecting from the replace context in this case. Is
this sufficient?
>
> /*
> * SINGLE profile on-disk has no profile bit, but in-memory we have a
>
