[PATCH RFC v4 0/3] block: enable RWF_DONTCACHE for block devices

[Tal Zussman <tz2294@columbia.edu>]

Add support for using RWF_DONTCACHE with block devices.

Dropbehind pruning needs to be done in non-IRQ context, but block
devices complete writeback in IRQ context.

To fix this, we can defer dropbehind invalidation to task context. We
introduce a new BIO_COMPLETE_IN_TASK flag that allows the bio submitter
to request task-context completion of bi_end_io. When bio_endio() sees
this flag in non-task context, it queues the bio to a per-CPU list and
schedules a work item to do bio completion.

Patch 1 adds the BIO_COMPLETE_IN_TASK infrastructure in the block
layer.

Patch 2 wires BIO_COMPLETE_IN_TASK into iomap writeback for DONTCACHE
folios and removes the DONTCACHE workqueue deferral from XFS.

Patch 3 enables RWF_DONTCACHE for block devices, setting
BIO_COMPLETE_IN_TASK in submit_bh_wbc() for the CONFIG_BUFFER_HEAD
path.

This support is useful for databases that operate on raw block devices,
among other userspace applications.

I tested this (with CONFIG_BUFFER_HEAD=y) for reads and writes on a
single block device on a VM, so results may be noisy.

Reads were tested on the root partition with a 45GB range (~2x RAM).
Writes were tested on a disabled swap parition (~1GB) in a memcg of size
244MB to force reclaim pressure.

Results:

===== READS (/dev/nvme0n1p2) =====
 sec   normal MB/s  dontcache MB/s
----  ------------  --------------
   1        1098.6          1609.0
   2        1270.3          1506.6
   3        1093.3          1576.5
   4        1141.8          2393.9
   5        1365.3          2793.8
   6        1324.6          2065.9
   7         879.6          1920.7
   8        1434.1          1662.4
   9        1184.9          1857.9
  10        1166.4          1702.8
  11        1161.4          1653.4
  12        1086.9          1555.4
  13        1198.5          1718.9
  14        1111.9          1752.2
----  ------------  --------------
 avg        1173.7          1828.8  (+56%)

==== WRITES (/dev/nvme0n1p3) =====
 sec   normal MB/s  dontcache MB/s
----  ------------  --------------
   1         692.4          9297.7
   2        4810.8          9342.8
   3        5221.7          2955.2
   4         396.7          8488.3
   5        7249.2          9249.3
   6        6695.4          1376.2
   7         122.9          9125.8
   8        5486.5          9414.7
   9        6921.5          8743.5
  10          27.9          8997.8
----  ------------  --------------
 avg        3762.5          7699.1  (+105%)

---
Changes in v4:
- 1/3: Move dropbehind deferral from folio-level to bio-level using
  BIO_COMPLETE_IN_TASK, per Matthew and Jan.
- 1/3: Work function yields on need_resched() to avoid hogging the CPU,
  per Jan.
- 2/3: New patch. Set BIO_COMPLETE_IN_TASK on iomap writeback bios for
  DONTCACHE folios, removing the need for XFS-specific workqueue
  deferral.
- 3/3: Set BIO_COMPLETE_IN_TASK in submit_bh_wbc() for buffer_head
  path.
- 3/3: Update commit message to mention CONFIG_BUFFER_HEAD=n path.
- Link to v3: https://lore.kernel.org/r/20260227-blk-dontcache-v3-0-cd309ccd5868@columbia.edu

Changes in v3:
- 1/2: Convert dropbehind deferral to per-CPU folio_batches protected by
  local_lock using per-CPU work items, to reduce contention, per Jens.
- 1/2: Call folio_end_dropbehind_irq() directly from
  folio_end_writeback(), per Jens.
- 1/2: Add CPU hotplug dead callback to drain the departing CPU's folio
  batch.
- 2/2: Introduce block_write_begin_iocb(), per Christoph.
- 2/2: Dropped R-b due to changes.
- Link to v2: https://lore.kernel.org/r/20260225-blk-dontcache-v2-0-70e7ac4f7108@columbia.edu

Changes in v2:
- Add R-b from Jan Kara for 2/2.
- Add patch to defer dropbehind completion from IRQ context via a work
  item (1/2).
- Add initial performance numbers to cover letter.
- Link to v1: https://lore.kernel.org/r/20260218-blk-dontcache-v1-1-fad6675ef71f@columbia.edu

---
Tal Zussman (3):
      block: add BIO_COMPLETE_IN_TASK for task-context completion
      iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback
      block: enable RWF_DONTCACHE for block devices

 block/bio.c                 | 84 ++++++++++++++++++++++++++++++++++++++++++++-
 block/fops.c                |  5 +--
 fs/buffer.c                 | 22 ++++++++++--
 fs/iomap/ioend.c            |  2 ++
 fs/xfs/xfs_aops.c           |  4 ---
 include/linux/blk_types.h   |  1 +
 include/linux/buffer_head.h |  3 ++
 7 files changed, 111 insertions(+), 10 deletions(-)
---
base-commit: 2961f841b025fb234860bac26dfb7fa7cb0fb122
change-id: 20260218-blk-dontcache-338133dd045e

Best regards,
-- 
Tal Zussman <tz2294@columbia.edu>