Re: [RFC V2 0/3] Add support to shrink multiple empty AGs

["Nirjhar Roy (IBM)" <nirjhar.roy.lists@gmail.com>]

On 9/16/25 20:34, Nirjhar Roy (IBM) wrote:
> This work is based on a previous RFC[1] by Gao Xiang and various ideas
> proposed by Dave Chinner in the RFC[1].
>
> Currently the functionality of shrink is limited to shrinking the last
> AG partially but not beyond that. This patch extends the functionality
> to support shrinking beyond 1 AG. However the AGs that we will be remove
> have to empty in order to prevent any loss of data.
>
> The patch begins with the re-introduction of some of the data
> structures that were removed, some code refactoring and
> finally the patch that implements the multi AG shrink design.
> The final patch has all the details including the definition of the
> terminologies and the overall design.
>
> We will have the tests soon.

Tests are here[1]

[1] 
https://lore.kernel.org/all/cover.1758035262.git.nirjhar.roy.lists@gmail.com/

--NR

>
> [rfc_v1] --> v2
> 1) Function renamings:
>      1.a xfs_activate_ag() -> xfs_perag_activate()
>      1.b xfs_deactivate_ag() -> xfs_perag_deactivate()
>      1.c xfs_pag_populate_cached_bufs() -> xfs_buf_cache_grab_all()
>      1.d xfs_buf_offline_perag_rele_cached() -> xfs_buf_cache_invalidate()
>      1.e xfs_extent_busy_wait_range() -> xfs_extent_busy_wait_ags()
>      1.f xfs_growfs_get_delta() -> xfs_growfs_compute_delta()
>
> 2) Fixed several coding style fixes and typos in the code and
>     commit messages.
>
> 3) Introduced for_each_perag_range_reverse() macro and used in
>     instead of using for loops directly.
>
> 4) Design changes:
>     4.a In function xfs_ag_is_empty() - Removed the
>         ASSERT(!xfs_ag_contains_log(mp, pag_agno(pag)));
>     4.b In function xfs_shrinkfs_reactivate_ags() - Replaced
>         if (nagcount >= oagcount) return; with ASSERT(nagcount < oagcount);
>     4.c In function xfs_perag_deactivate() - Add one extra step where
>         we manually reduce/reserve (pagf_freeblks + pagf_flcount) worth of
>         free datablocks from the global counters. This is necessary
>         in order to prevent a race where, some AGs have been temporarily
>         offlined but the delayed allocator has already promised some bytes
>         and later the real extent/block allocation is failing due to
>         the AG(s) being offline.
>     4.d In function xfs_perag_activate() - Add one extra step where
>         we restore the global free block counter which we reduced in
>         xfs_perag_deactivate.
>     4.e In function xfs_shrinkfs_deactivate_ags() -
>             1. Flushing the xfs_discard_wq after the log force/flush.
> 	   2. Removed the direct usage of xfs_log_quiesce(). The reason
> 	      is that xfs_log_quiesce() is expected to be called when the
> 	      caller has made sure that the log/filesystem is idle but
> 	      for shrink, we don't necessarily need the log/filesystem
> 	      to be idle.
> 	      However, we still need the checkpointing to take place,
> 	      so we are doing a xfs_sync_sb+AIL flush twice - something
> 	      similar that is being done in xfs_log_cover().
> 	      More details are in the patch.
>             3. Moved the entire code of ag stabilization (after ag
> 	      offlining) into a separate function -
> 	      xfs_shrinkfs_stabilize_ags().
>     4.f Fixed a bug where if the size of the new tail AG was less than
>         XFS_MIN_AG_BLOCKS, then shrink was passing - the correct behavior
>         is to fail with -EINVAL. Thank you Ritesh[2] for pointing this out.
>
> 5) Added RBs from Darrick in patch 1/3 and patch 2/3 (after addressing his
>     comments).
>
> [1] https://lore.kernel.org/all/20210414195240.1802221-1-hsiangkao@redhat.com/
> [2] https://lore.kernel.org/all/875xfas2f6.fsf@gmail.com/
> [rfc_v1] https://lore.kernel.org/all/cover.1752746805.git.nirjhar.roy.lists@gmail.com/
>
> Nirjhar Roy (IBM) (3):
>    xfs: Re-introduce xg_active_wq field in struct xfs_group
>    xfs: Refactoring the nagcount and delta calculation
>    xfs: Add support to shrink multiple empty AGs
>
>   fs/xfs/libxfs/xfs_ag.c        | 193 +++++++++++++++++-
>   fs/xfs/libxfs/xfs_ag.h        |  17 ++
>   fs/xfs/libxfs/xfs_alloc.c     |   9 +-
>   fs/xfs/libxfs/xfs_group.c     |   4 +-
>   fs/xfs/libxfs/xfs_group.h     |   2 +
>   fs/xfs/xfs_buf.c              |  78 ++++++++
>   fs/xfs/xfs_buf.h              |   1 +
>   fs/xfs/xfs_buf_item_recover.c |  37 ++--
>   fs/xfs/xfs_extent_busy.c      |  30 +++
>   fs/xfs/xfs_extent_busy.h      |   2 +
>   fs/xfs/xfs_fsops.c            | 358 +++++++++++++++++++++++++++++++---
>   fs/xfs/xfs_trans.c            |   1 -
>   12 files changed, 678 insertions(+), 54 deletions(-)
>
> --
> 2.43.5
>
-- 
Nirjhar Roy
Linux Kernel Developer
IBM, Bangalore