Re: [PATCH] btrfs: derive f_fsid with dev_t only when temp_fsid is active

[Anand Suveer Jain <asj@kernel.org>]

On 15/9/26 00:46, David Sterba wrote:
> On Mon, Sep 14, 2026 at 11:42:51AM +0800, Anand Suveer Jain wrote:
>>
>>
>> This issue and its current fix (below) have been on my mind, as I wasn't
>> fully satisfied with the approach even though it works.
>>
>> Now, I think I have a better idea.
>>
>> The core issue isn't that the new f_fsid derivation is wrong (recap:
>> f_fsid derivation changed from f(UUID) to f(UUID + dev_t)). Rather, the
>> problem is that f_fsid changes unexpectedly after an upgrade.
>>
>> So we need to address the upgrade behavior, not how f_fsid itself is
>> derived.
>>
>> My new proposed solution:
>>
>>  . Put the new f_fsid derivation behind a compile-time config flag (an
>> incompat/opt-in flag).
> 
> A compile-time change should be only something really significant for
> the whole filesystem. Configs rarely change and except
> developers/embedded/custom builds it's hard to change, speaking about
> distros.

I'm responding to other emails collectively here:

In that case, I think it's better to revert the commit c2a74ed0494c
("btrfs: derive f_fsid from on-disk fsid and dev_t"). That gives us
consistent statfs f_fsid behavior in the new kernel and for
temp_fsid-mounted filesystems, f_fsid will be the random value generated
at mount time, and it becomes the responsibility of the use case to
change the UUID (and move away from relying on temp_fsid) if persistent
identification is required. btrfs(5) is updated [1] in the ML.

 [1]
https://lore.kernel.org/linux-btrfs/20260918084612.ZxGVtNO9DAJfevT0EwV5BlMy-4qgIyRKK6TAZ5oNLtU@z/