Re: [PATCH 5/7] btrfs: zoned: subtract zone_unusable space in statfs

[Christoph Hellwig <hch@lst.de>]

On Fri, May 15, 2026 at 11:26:22AM +0200, Johannes Thumshirn wrote:
> On 5/15/26 6:39 AM, Christoph Hellwig wrote:
>> On Wed, May 13, 2026 at 02:34:43PM +0200, Johannes Thumshirn wrote:
>>> On zoned filesystems, space in block groups that has been freed but not
>>> yet reset is tracked in bytes_zone_unusable. This space cannot be used for
>>> new allocations until zone reclaim resets the zones, but it was being
>>> reported as available space in statfs.
>>>
>>> This caused statfs to over-report free space, leading to ENOSPC errors
>>> when applications tried to allocate based on the reported free space.
>>>
>>> Fix this by subtracting bytes_zone_unusable from total_free_data in the
>>> statfs calculation for zoned filesystems.
>> This OTOH sounds very wrong.  Freed but not reclaimed space is usable,
>> it just needs work.
>>
> man statfs says:
>
>                fsblkcnt_t f_bavail;  /* Free blocks available to
>                                         unprivileged user */
>
> which in my interpretation would include zone_unusable_bytes, as they're 
> not "free blocks available". Or would that be accounted as f_bfree?

The only different between f_bavail and f_bfree is reservations for
the superuser, which IIRC only extN do on Linux.

f_bavail is all the space you as a user could potentially use on
the file system (modulo quota restrictions).