Re: [PATCH 0/7] btrfs: fixes around generic/747 on zoned filesystems

[Boris Burkov <boris@bur.io>]

On Wed, May 13, 2026 at 02:34:38PM +0200, Johannes Thumshirn wrote:
> This series fixes premature ENOSPC errors and starvation issues on zoned BTRFS
> filesystems when running xfstest generic/747, which tests garbage collection
> on zoned block devices using direct and buffered I/O.
> 
> The investigation revealed two distinct issues:
> 
> 1. Async reclaim starvation: On zoned filesystems, the flush state machine
>    only executes RECLAIM_ZONES and RESET_ZONES in later flush states
>    (FLUSH_DELALLOC and beyond). By the time these states are reached,
>    ticket waiters can starvation waiting for space that can only be freed

"can starvation" reads like a typo or lost sentence edit

>    by zone reset. The fix adds RECLAIM_ZONES and RESET_ZONES to the first
>    async reclaim loop (FLUSH_ALLOC) specifically for zoned filesystems,
>    ensuring zone reset happens early enough to free space for pending
>    allocation tickets.
> 
> 2. Inaccurate statfs reporting: 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. The fix subtracts bytes_zone_unusable
>    from total_free_data in the statfs calculation for zoned filesystems,
>    with proper RAID factor multiplication for unit conversion.
> 
> Additionally, the series fixes a bug in data relocation block group selection
> where the first block group was incorrectly skipped, and adds a new flush
> state (BTRFS_RESERVE_FLUSH_DATA_RELOCATION) to use priority reclaim for
> zoned data relocation operations.
> 
> 
> Johannes Thumshirn (7):
>   btrfs: zoned: document RECLAIM_ZONES flush state
>   btrfs: zoned: decode 'RECLAIM_ZONES' state in tracepoints
>   btrfs: zoned: always set data_relocation_bg
>   btrfs: zoned: don't account data relocation space-info in statfs free
>     space
>   btrfs: zoned: subtract zone_unusable space in statfs
>   btrfs: zoned: fix deadlock waiting for ticket during data relocation
>   btrfs: zoned: add RECLAIM_ZONES and RESET_ZONES to first async reclaim
>     loop
> 
>  fs/btrfs/delalloc-space.c    |  2 ++
>  fs/btrfs/space-info.c        | 18 ++++++++++++++++++
>  fs/btrfs/space-info.h        | 11 +++++++++++
>  fs/btrfs/super.c             |  5 ++++-
>  fs/btrfs/zoned.c             |  6 +++---
>  include/trace/events/btrfs.h |  1 +
>  6 files changed, 39 insertions(+), 4 deletions(-)
> 
> -- 
> 2.54.0
>