[LSF/MM/BPF TOPIC] Large folio support: iomap framework changes versus filesystem-specific implementations

["Nanzhe Zhao" <nzzhao@126.com>]

Large folios can reduce per-page overhead and improve throughput for large buffered I/O, but enabling them in filesystems is not a mechanical “page → folio” conversion. The core difficulty is preserving correctness and performance when a folio must maintain subrange state, while existing filesystem code paths and the iomap buffered I/O framework make different assumptions about state tracking, locking lifetime, block mapping, and writeback semantics.

This session proposes a cross-filesystem discussion around two directions that are actively being explored:

Iomap approach: adopt iomap buffered I/O paths and benefit from iomap-style subrange folio state machinery. However, much of this machinery lives as static helpers inside iomap’s implementation (e.g., in buffered-io.c) and is not available as a reusable API, which pushes filesystems toward re-implementing similar logic. Moreover, iomap’s per-folio state relies on folio-private metadata storage, which can clash with filesystem-specific folio-private usage.


Native fs approach: keep native buffered I/O paths and implement filesystem-specific folio_state tracking and helpers to avoid whole-folio dirtying/write amplification and to match filesystem-private metadata (e.g., private flags). This avoids some iomap integration constraints and preserves filesystem-specific optimizations, but it increases filesystem-local complexity and long-term maintenance cost.


Using f2fs as a concrete instance (log-structured, indirect-pointer mapping, private folio flags), this session consolidates two recurring issues relevant across filesystems:

Per-folio state tracking: iomap subrange-state API exposure vs filesystem-local solution.
COW writeback support: minimal iomap extensions vs filesystem-local writeback for COW paths.

The goal is to converge on recommended design patterns and actionable next steps for f2fs/ext4/btrfs/others to enable large folios without correctness risks or performance regressions.

Best regards,
Nanzhe Zhao

Related Patches for Large Folios:

f2fs:
- https://lore.kernel.org/all/20250813092131.44762-1-nzzhao@126.com/
- https://lore.kernel.org/linux-f2fs-devel/20251120235446.1947532-1-jaegeuk@kernel.org/
- https://lore.kernel.org/linux-f2fs-devel/20260203091256.854842-1-nzzhao@126.com/

ext4:
- https://lore.kernel.org/all/20250512063319.3539411-1-yi.zhang@huaweicloud.com/

btrfs:
- https://lore.kernel.org/all/676154e5415d8d15499fb8c02b0eabbb1c6cef26.1745403878.git.wqu@suse.com/