Add support for using RWF_DONTCACHE with block devices and other
buffer_head-based I/O.

Dropbehind pruning needs to be done in non-IRQ context, but block
devices complete writeback in IRQ context. To fix this, we first defer
dropbehind completion initiated from IRQ context by scheduling a work
item to process a per-CPU batch of folios.

Then, add a block_write_begin_iocb() variant that threads the kiocb
through for RWF_DONTCACHE I/Os.

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         993.9          1799.6
   2         992.8          1693.8
   3         923.4          2565.9
   4        1013.5          3917.3
   5        1557.9          2438.2
   6        2363.4          1844.3
   7        1447.9          2048.6
   8         899.4          1951.7
   9        1246.8          1756.1
  10        1139.0          1665.6
  11        1089.7          1707.7
  12        1270.4          1736.5
  13        1244.0          1756.3
  14        1389.7          1566.2
----  ------------  --------------
 avg        1258.0          2005.4  (+59%)

==== WRITES (/dev/nvme0n1p3) =====
 sec   normal MB/s  dontcache MB/s
----  ------------  --------------
   1        2396.1          9670.6
   2        8444.8          9391.5
   3         770.8          9400.8
   4          61.5          9565.9
   5        7701.0          8832.6
   6        8634.3          9912.9
   7         469.2          9835.4
   8        8588.5          9587.2
   9        8602.2          9334.8
  10         591.1          8678.8
  11        8528.7          3847.0
----  ------------  --------------
 avg        4981.7          8914.3  (+79%)

---
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 (2):
      filemap: defer dropbehind invalidation from IRQ context
      block: enable RWF_DONTCACHE for block devices

 block/fops.c                |   5 +-
 fs/buffer.c                 |  19 ++++++-
 include/linux/buffer_head.h |   3 +
 include/linux/pagemap.h     |   1 +
 mm/filemap.c                | 130 +++++++++++++++++++++++++++++++++++++++++---
 mm/page_alloc.c             |   1 +
 6 files changed, 145 insertions(+), 14 deletions(-)
---
base-commit: 05f7e89ab9731565d8a62e3b5d1ec206485eeb0b
change-id: 20260218-blk-dontcache-338133dd045e

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