Re: [PATCH v3 0/4] btrfs: reduce btrfs_get_extent() calls for buffered write path

[Boris Burkov <boris@bur.io>]

On Thu, Nov 20, 2025 at 07:46:45PM +1030, Qu Wenruo wrote:
> [CHANGELOG]
> v3:
> - Use @cur_len for btrfs_get_extent() in the last patch
>   And remove the hole related comment.
>   This should not cause any behavior change.
> 
>   The @len parameter for btrfs_get_extent() is only going to cause a
>   difference when the range is completely a hole (e.g. beyond EOF).
> 
>   For our writeback routine, there should be an extent map for us thus
>   it's no different passing @cur_len or sectorsize.
> 
>   I think this @len parameter of btrfs_get_extent() is causing
>   unnecessary complexity, and want to remove it completely. But that
>   will be a new series.
> 
> v2:
> - Fix a potential bug where OEs beyond EOF are not truncated properly
>   This replace the original patch to extract the code into a helper.
> 
> - Replace more for_each_set_bit() with for_each_set_bitrange()
> 
> - Fix several copy-n-pasted incorrect range inside submit_range()
> 
> 
> Although btrfs has bs < ps support for a long time, and the larger data
> folios support is also going to be graduate from experimental features
> soon, the write path is still iterating each fs block and call
> btrfs_get_extent() on each fs block.
> 
> What makes the situation worse is that, for the write path we do not
> have any cached extent map, meaning even with large folios and we got a
> continuous range that can be submitted in one go, we still call
> btrfs_get_extent() many times and get the same range extent map again
> and again.
> 
> This series will reduce the duplicated btrfs_get_extent() calls by only
> call it once for each range, other than for each fs block.
> 
> The first one is a potential bug inspired by Boris' review.
> Patch 2~3 are minor cleanups.
> Patch 4 is the core of the optimization.
> 
> Although I don't expect there will be much difference in the real world though.

Reviewed-by: Boris Burkov <boris@bur.io>

> 
> 
> Qu Wenruo (4):
>   btrfs: make sure all ordered extents beyond EOF is properly truncated
>   btrfs: integrate the error handling of submit_one_sector()
>   btrfs: replace for_each_set_bit() with for_each_set_bitmap()
>   btrfs: reduce extent map lookup during writes
> 
>  fs/btrfs/extent_io.c    | 230 ++++++++++++++++++++--------------------
>  fs/btrfs/ordered-data.c |  38 +++++++
>  fs/btrfs/ordered-data.h |   2 +
>  3 files changed, 156 insertions(+), 114 deletions(-)
> 
> -- 
> 2.52.0
>