Re: [PATCH 1/2] btrfs: drop never met condition of disk_total_bytes == 0

[Nikolay Borisov <nborisov@suse.com>]

On 25.09.20 г. 7:17 ч., Anand Jain wrote:
> 
> 
> On 24/9/20 7:48 pm, Nikolay Borisov wrote:
>>
>>
>> On 24.09.20 г. 13:11 ч., Anand Jain wrote:
>>> btrfs_device::disk_total_bytes is set even for a seed device (the
>>> comment is wrong).
>>>
>>> The function fill_device_from_item() does the job of reading it from the
>>> item and updating btrfs_device::disk_total_bytes. So both the missing
>>> device and the seed devices do have their disk_total_bytes updated.
>>>
>>> So this patch removes the check dev->disk_total_bytes == 0 in the
>>> function verify_one_dev_extent()
>>>
>>> Signed-off-by: Anand Jain <anand.jain@oracle.com>
>>> ---
>>>   fs/btrfs/volumes.c | 15 ---------------
>>>   1 file changed, 15 deletions(-)
>>>
>>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>>> index 7f43ed88fffc..9be40eece8ed 100644
>>> --- a/fs/btrfs/volumes.c
>>> +++ b/fs/btrfs/volumes.c
>>> @@ -7578,21 +7578,6 @@ static int verify_one_dev_extent(struct
>>> btrfs_fs_info *fs_info,
>>>           goto out;
>>>       }
>>>   -    /* It's possible this device is a dummy for seed device */
>>> -    if (dev->disk_total_bytes == 0) {
>>> -        struct btrfs_fs_devices *devs;
>>> -
>>> -        devs = list_first_entry(&fs_info->fs_devices->seed_list,
>>> -                    struct btrfs_fs_devices, seed_list);
>>> -        dev = btrfs_find_device(devs, devid, NULL, NULL, false);
>>> -        if (!dev) {
>>> -            btrfs_err(fs_info, "failed to find seed devid %llu",
>>> -                  devid);
>>> -            ret = -EUCLEAN;
>>> -            goto out;
>>> -        }
>>> -    }
>>
>> The commit which introduced this check states that the device with a
>> disk_total_bytes = 0 occurs from clone_fs_devices called from open_seed.
>> It seems the check is legit and your changelog doesn't account for that
>> if it's safe you should provide description why is that.
> 
> yes the commit 1b3922a8bc74 (btrfs: Use real device structure to verify
> dev extent) introduced it. In Qu's analysis, it is unclear why the
> total_disk_bytes was 0.
> 
> Theoretically, all devices (including missing and seed) marked with the
> BTRFS_DEV_STATE_IN_FS_METADATA flag gets the total_disk_bytes updated at
> fill_device_from_item().
> 
> open_ctree()
>  btrfs_read_chunk_tree()
>   read_one_dev()
>    open_seed_device() 

This function returns the cloned fs_devices whose devices are not
initialized. Later, in read_one_dev the 'device' is acquired from
fs_info->fs_devices, not from the returned fs_devices :

device = btrfs_find_device(fs_info->fs_devices, devid, dev_uuid,
                           fs_uuid, true);

And finally fill_device_from_item(leaf, dev_item, device); is called for
the device which was found from fs_info->fs_devices and not from the
returned 'fs_devices' from :

fs_devices = open_seed_devices(fs_info, fs_uuid);

What this means is that struct btrfs_device of devices in
fs_info->seed_list is never fully initialized.

>    fill_device_from_item()
> 
> Even if verify_one_dev_extent() reports total_disk_bytes == 0, then IMO
> its a bug to be fixed somewhere else and not in verify_one_dev_extent()
> as it's just a messenger. It is never expected that a total_disk_bytes
> shall be zero.

I agree, however this would involve fixing clone_fs_devices to properly
initialize struct btrfs_device. I'm in favor of removing special casing.

Looking closer into verify_one_dev_extent I see that the device is
referenced from fs_info->fs_devices :

dev = btrfs_find_device(fs_info->fs_devices, devid, NULL, NULL, true);

And indeed, it seems that all devices of fs_info->fs_devices are
initialized as per your above explanation. So yeah, your patch is
codewise correct but the changelog is wrong:

disk_total_bytes is never set for seed devices (seed devices are after
all housed in fs_info->seed_list which as I explained above doesn't
fully initialize btrfs_devices)

A better changelog would document following invariants:

1. Seed devices don't have their disktotal bytes initialized

2. In spite of (1), verify_dev_one_extent is never called for such
devices so it's fine because devices anchored at fs_info->fs_devices are
always properly initialized.


<snip>