Re: [PATCH v2 0/4] allocate extent_buffer GFP_NOFAIL with unlocked retry

On Wed, Jul 22, 2026 at 3:04 AM Boris Burkov <[email protected]> wrote:
>
> From sampled fleet data measuring lock holders that go into direct
> reclaim with a waiter present when they eventually unlock, we have
> observed ~15% of those are btrfs extent_buffer allocations in
> btrfs_search_slot() done while holding btree locks. This is the single
> largest category. Additionally, a large source of hung_task timeouts is
> both btree waiters and direct reclaiming btree allocations, which
> further motivates the desire to drive down this source of stalls and
> contention.
>
> The aim of this series is to allow us to allocate the extent_buffer,
> btrfs_folio_state, and the extent_buffer folios with GFP_NOWAIT then
> fallback with EAGAIN to outside the critical section to retry with
> GFP_NOFS | GFP_NOFAIL without any locks held.
>
> This is analogous to how we must drop locks to read an extent_buffer and
> then EAGAIN.
>
> The series does not manage to completely eliminate allocations from this
> lock holding path, as we also allocate inside xarray functions for the
> extent_buffer xarray and the btree_inode mapping xarray, the latter of
> which is done via filemap_add_folio() with no reserve type API. Luckily,
> those particular allocations are small cached slab allocations and have
> nearly no contribution to the production reclaim fueled contention.
>
> Changelog:
> v2:
> - rebase
> - dropped the inhibit array allocation patch
>
>
> Boris Burkov (4):
>   btrfs: factor init_extent_buffer from __alloc_extent_buffer
>   btrfs: add struct btrfs_eb_prealloc
>   btrfs: enable unlocked NOFAIL retry for eb allocations
>   btrfs: use GFP_NOWAIT for tree block readahead

Apart from the comments left in the second patch:

Reviewed-by: Filipe Manana <[email protected]>

Thanks.

>
>  fs/btrfs/ctree.c       |  36 ++++++-
>  fs/btrfs/disk-io.c     |   6 +-
>  fs/btrfs/disk-io.h     |   2 +
>  fs/btrfs/extent-tree.c |   6 +-
>  fs/btrfs/extent_io.c   | 213 ++++++++++++++++++++++++++++-------------
>  fs/btrfs/extent_io.h   |  23 +++++
>  fs/btrfs/subpage.c     |   7 +-
>  fs/btrfs/subpage.h     |   3 +-
>  fs/btrfs/tree-log.c    |   3 +-
>  9 files changed, 220 insertions(+), 79 deletions(-)
>
> --
> 2.55.0
>
>