Re: first mount(s) after unclean shutdown always fail

[Marc Lehmann <schmorp@schmorp.de>]

On Thu, Jul 02, 2020 at 08:02:52AM +0800, Qu Wenruo <quwenruo.btrfs@gmx.com> wrote:
> Well, if you want to go this way, let me show the code here.
> 
> From fs/btrfs/volumes.c:btrfs_read_chunk_tree():
> 
>         if (btrfs_super_total_bytes(fs_info->super_copy) <
>             fs_info->fs_devices->total_rw_bytes) {
>                 btrfs_err(fs_info,
>         "super_total_bytes %llu mismatch with fs_devices total_rw_bytes
> %llu",
>                           btrfs_super_total_bytes(fs_info->super_copy),
>                           fs_info->fs_devices->total_rw_bytes);
>                 ret = -EINVAL;
>                 goto error;
>         }
> 
> Doesn't this explain why we abort the mount?

I wouldn't see how, especially if the code doesn't do anything _unless_ it
also prints the message.

When it doesn't produce the message, all it does is compare two numbers
(unless btrfs_super_total_bytes does something very funny) - how does this
explain that the mount fails, then succeeds, in the cases where the message
is _not_ logged, as reported?

> > Also, shouldn't btrfs be fixed instead? I was under the impression that
> > one of the goals of btrfs is to be safe w.r.t. crashes.
> 
> That's why we provide the btrfs rescue fix-device-size.

Not sure how that follows - there is a bug in the kernel filesystem and
you provide a userspace tool that should be run on every crash, to what
end?

Spurious mount failures are a bug in the btrfs kernel driver.

> > The bug I reported has very little or nothing to with strict checking.
> 
> I have provide the code to prove why it's related.

The code proves only that you are wrong - the code _always_ prints the
message. Unless btrfs_super_total_bytes does more than just read some
data, it cannot explain the bug I reported, simply because the message is
not always produced, and the mount is not always aborted.

> Whether you believe is your problem then.

No, it's not, simply because I don't have a problem...

btrfs has problems, and I reported one, that's all that has happened.

I slowly get the distinct feeling that reporting bugs in btrfs us a futile
exercise, though.

-- 
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