[PATCH v3 0/2] f2fs: enable buffered RWF_DONTCACHE

[Wenjie Qi <qwjhust@gmail.com>]

This series enables buffered RWF_DONTCACHE on F2FS for sustained one-pass
streaming writes, where retaining the written data can displace more useful
cache.

Patch 1 records whether an F2FS write bio contains dropbehind folios, keeps
normal and dropbehind folios in separate bios in the IPU and OPU paths, and
defers unsafe dropbehind completion through the existing sbi->wq.

Patch 2 passes FGP_DONTCACHE to the F2FS buffered write folio lookup and
advertises FOP_DONTCACHE.

Tests were run on a Xiaomi phone with 10.7 GiB of kernel-visible memory,
running Android 16 and Linux 6.12.69 with 4 KiB pages.  /data used F2FS.

The performance test wrote exactly 64 GiB per run at 4 KiB,
8 KiB, 16 KiB, 32 KiB, 64 KiB, 128 KiB, 256 KiB, 512 KiB, and 1 MiB.
Two counterbalanced rounds ran ascending normal-first and descending
dontcache-first.  Values below are equal-weight means of both runs; N=2.
The pwritev2() writer models the streaming workload; it does not show that
an unchanged Android application already issues RWF_DONTCACHE.

Android remained active with displays off.  Each run started after a cache
reset and at least 120 seconds of cooldown.  Throughput and one-second
kswapd0/global-memory samples cover the write loop.

Write-loop throughput was:

          normal MiB/s              dontcache MiB/s
 I/O      r1       r2      mean      r1       r2      mean     change
 4K      946.28   935.74   941.01   291.36   309.31   300.33   -68.08%
 8K     1077.05  1105.84  1091.45   477.79   479.79   478.79   -56.13%
 16K    1126.84  1118.49  1122.67   643.40   652.06   647.73   -42.30%
 32K    1150.62  1036.33  1093.48   762.24   751.45   756.84   -30.79%
 64K    1144.80  1163.82  1154.31   852.11   851.19   851.65   -26.22%
 128K   1166.29  1162.84  1164.57   867.47   865.05   866.26   -25.61%
 256K   1153.61  1172.78  1163.19   895.53   885.33   890.43   -23.45%
 512K   1173.61  1197.34  1185.48   903.09   903.01   903.05   -23.82%
 1M     1126.22  1154.59  1140.41   850.74   894.60   872.67   -23.48%

Average kswapd0 CPU and average global Cached were:

 I/O      kswapd0 CPU, normal/DC        Cached MiB, normal/DC
 4K             18.45% / 0%                 4830.15 / 686.56
 8K             21.60% / 0%                 4862.35 / 591.48
 16K            22.75% / 0%                 4905.47 / 625.07
 32K            22.05% / 0%                 4945.82 / 561.59
 64K            23.46% / 0%                 4920.77 / 639.78
 128K           23.16% / 0%                 4971.37 / 693.26
 256K           23.68% / 0%                 4956.93 / 668.60
 512K           24.25% / 0%                 4972.22 / 663.92
 1M             22.01% / 0%                 5001.09 / 705.30

Other global memory means were:

          MemAvailable MiB       Dirty MiB          Writeback MiB
 I/O       normal / DC          normal / DC          normal / DC
 4K      6513.08 / 6382.31     640.48 / 33.93       37.94 / 0.09
 8K      6560.77 / 6529.33     690.89 / 43.57       40.84 / 0.54
 16K     6587.46 / 6519.94     789.16 / 62.67       61.58 / 4.64
 32K     6571.95 / 6566.74     850.48 / 69.14       67.69 / 9.24
 64K     6600.64 / 6559.89    856.97 / 134.28      59.60 / 16.19
 128K    6627.03 / 6465.10    873.68 / 137.57      60.57 / 41.50
 256K    6615.58 / 6541.09    885.98 / 158.15      61.25 / 29.57
 512K    6625.52 / 6534.41    900.65 / 139.07      63.65 / 30.35
 1M      6677.09 / 6539.89    909.70 / 187.56      51.58 / 33.46

          Active(file) MiB      Inactive(file) MiB
 I/O       normal / DC             normal / DC
 4K       279.57 / 264.13         4426.38 / 183.12
 8K       262.32 / 260.97         4472.62 / 182.19
 16K      392.09 / 252.95         4392.78 / 195.82
 32K      254.30 / 248.44         4554.18 / 187.34
 64K      252.73 / 244.94         4545.40 / 267.28
 128K     322.02 / 243.95         4530.63 / 298.91
 256K     245.47 / 240.02         4586.64 / 303.92
 512K     254.75 / 232.05         4591.17 / 288.27
 1M       238.88 / 233.64         4638.81 / 341.00

Dontcache left zero target-file pages resident at every size.  Normal
retained about 1.19--1.24 million pages.  Normal runs incurred roughly
15.6 million kswapd page scans and steals per run, while dontcache recorded
zero.  Direct scan and allocation-stall deltas were zero in both modes.

A controlled explicit-dontcache model issued 64 KiB writes for 120 seconds
at 64, 128, and 256 MiB/s.  Both modes sustained all three rates in both
rounds with no final schedule overrun.  The late-write ratio was
0.02%--0.41%, and maximum schedule lag was 3.3--6.0 ms.
Dontcache left zero target pages resident.  This was a controlled model,
not an unchanged Xiaomi application.

Read tests were unpaced.  Each run started with zero source pages resident
and read the same 64 GiB file sequentially at full speed with preadv2().
Results from two counterbalanced rounds were:

 I/O       normal MiB/s   dontcache MiB/s   change
 4K          1825.50          1695.10        -7.14%
 8K          1901.46          1847.96        -2.81%
 16K         1941.60          1872.55        -3.56%
 32K         1960.95          1905.92        -2.81%
 64K         1951.35          1886.54        -3.32%
 128K        1960.03          1915.82        -2.26%
 256K        1976.01          1900.41        -3.83%
 512K        1973.31          1911.24        -3.15%
 1M          2230.91          2007.82       -10.00%

Dontcache left zero source pages resident in all measured read runs.

The normal-I/O control showed read-throughput differences of +0.67%,
-0.90%, and -2.58%, and write-throughput differences of -1.75%, +0.33%,
and +2.33%, at 4 KiB, 64 KiB, and 1 MiB respectively.

Changes since v2:

- keep the implementation unchanged;
- add paced-write, unpaced-read, and normal-I/O control results.

Wenjie Qi (2):
  f2fs: complete dropbehind write bios in safe task context
  f2fs: enable buffered RWF_DONTCACHE

 fs/f2fs/data.c | 59 ++++++++++++++++++++++++++++++++++++++++----------
 fs/f2fs/file.c |  2 +-
 2 files changed, 49 insertions(+), 12 deletions(-)

-- 
2.43.0