[PATCH 00/27] ext4: use iomap for regular file's buffered I/O path and enable large folio

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From: Zhang Yi <yi.zhang@xxxxxxxxxx>

Hello!

This patch series is the latest version based on my previous RFC
series[1], which converts the buffered I/O path of ext4 regular files to
iomap and enables large folios. After several months of work, almost all
preparatory changes have been upstreamed, thanks a lot for the review
and comments from Jan, Dave, Christoph, Darrick and Ritesh. Now it is
time for the main implementation of this conversion. 

This series is the main part of iomap buffered iomap conversion, it's
based on 6.12-rc4, and the code context is also depend on my anohter
cleanup series[1] (I've put that in this seris so we can merge it
directly), fixed all minor bugs found in my previous RFC v4 series.
Additionally, I've update change logs in each patch and also includes
some code modifications as Dave's suggestions. This series implements
the core iomap APIs on ext4 and introduces a mount option called
"buffered_iomap" to enable the iomap buffered I/O path. We have already
supported the default features, default mount options and bigalloc
feature. However, we do not yet support online defragmentation, inline
data, fs_verify, fs_crypt, ext3, and data=journal mode, ext4 will fall
to buffered_head I/O path automatically if you use those features and
options. Some of these features should be supported gradually in the
near future.

Most of the implementations resemble the original buffered_head path;
however, there are four key differences.

1. The first aspect is the block allocation in the writeback path. The
   iomap frame will invoke ->map_blocks() at least once for each dirty
   folio. To ensure optimal writeback performance, we aim to allocate a
   range of delalloc blocks that is as long as possible within the
   writeback length for each invocation. In certain situations, we may
   allocate a range of blocks that exceeds the amount we will actually
   write back. Therefore,
1) we cannot allocate a written extent for those blocks because it may
   expose stale data in such short write cases. Instead, we should
   allocate an unwritten extent, which means we must always enable the
   dioread_nolock option. This change could also bring many other
   benefits.
2) We should postpone updating the 'i_disksize' until the end of the I/O
   process, based on the actual written length. This approach can also
   prevent the exposure of zero data, which may occur if there is a
   power failure during an append write.
3) We do not need to pre-split extents during write-back, we can
   postpone this task until the end I/O process while converting
   unwritten extents.

2. The second reason is that since we always allocate unwritten space
   for new blocks, there is no risk of exposing stale data. As a result,
   we do not need to order the data, which allows us to disable the
   data=ordered mode. Consequently, we also do not require the reserved
   handle when converting the unwritten extent in the final I/O worker,
   we can directly start with the normal handle.

Series details:

Patch 1-10 is just another series of mine that refactors the fallocate
functions[1]. This series relies on the code context of that but has no
logical dependencies. I put this here just for easy access and merge.

Patch 11-21 implement the iomap buffered read/write path, dirty folio
write back path and mmap path for ext4 regular file. 

Patch 22-23 disable the unsupported online-defragmentation function and
disable the changing of the inode journal flag to data=journal mode.
Please look at the following patch for details.

Patch 24-27 introduce "buffered_iomap" mount option (is not enabled by
default now) to partially enable the iomap buffered I/O path and also
enable large folio.


About performance:

Fio tests with psync on my machine with Intel Xeon Gold 6240 CPU with
400GB system ram, 200GB ramdisk and 4TB nvme ssd disk.

 fio -directory=/mnt -direct=0 -iodepth=$iodepth -fsync=$sync -rw=$rw \
     -numjobs=${numjobs} -bs=${bs} -ioengine=psync -size=$size \
     -runtime=60 -norandommap=0 -fallocate=none -overwrite=$overwrite \
     -group_reportin -name=$name --output=/tmp/test_log

 == buffer read ==

                buffer_head        iomap + large folio
 type     bs    IOPS    BW(MiB/s)  IOPS    BW(MiB/s)
 -------------------------------------------------------
 hole     4K    576k    2253       762k    2975     +32%
 hole     64K   48.7k   3043       77.8k   4860     +60%
 hole     1M    2960    2960       4942    4942     +67%
 ramdisk  4K    443k    1732       530k    2069     +19%
 ramdisk  64K   34.5k   2156       45.6k   2850     +32%
 ramdisk  1M    2093    2093       2841    2841     +36%
 nvme     4K    339k    1323       364k    1425     +8%
 nvme     64K   23.6k   1471       25.2k   1574     +7%
 nvme     1M    2012    2012       2153    2153     +7%


 == buffer write ==

                                       buffer_head  iomap + large folio
 type   Overwrite Sync Writeback  bs   IOPS   BW    IOPS   BW(MiB/s)
 ----------------------------------------------------------------------
 cache      N    N    N    4K     417k    1631    440k    1719   +5%
 cache      N    N    N    64K    33.4k   2088    81.5k   5092   +144%
 cache      N    N    N    1M     2143    2143    5716    5716   +167%
 cache      Y    N    N    4K     449k    1755    469k    1834   +5%
 cache      Y    N    N    64K    36.6k   2290    82.3k   5142   +125%
 cache      Y    N    N    1M     2352    2352    5577    5577   +137%
 ramdisk    N    N    Y    4K     365k    1424    354k    1384   -3%
 ramdisk    N    N    Y    64K    31.2k   1950    74.2k   4640   +138%
 ramdisk    N    N    Y    1M     1968    1968    5201    5201   +164%
 ramdisk    N    Y    N    4K     9984    39      12.9k   51     +29%
 ramdisk    N    Y    N    64K    5936    371     8960    560    +51%
 ramdisk    N    Y    N    1M     1050    1050    1835    1835   +75%
 ramdisk    Y    N    Y    4K     411k    1609    443k    1731   +8%
 ramdisk    Y    N    Y    64K    34.1k   2134    77.5k   4844   +127%
 ramdisk    Y    N    Y    1M     2248    2248    5372    5372   +139%
 ramdisk    Y    Y    N    4K     182k    711     186k    730    +3%
 ramdisk    Y    Y    N    64K    18.7k   1170    34.7k   2171   +86%
 ramdisk    Y    Y    N    1M     1229    1229    2269    2269   +85%
 nvme       N    N    Y    4K     373k    1458    387k    1512   +4%
 nvme       N    N    Y    64K    29.2k   1827    70.9k   4431   +143%
 nvme       N    N    Y    1M     1835    1835    4919    4919   +168%
 nvme       N    Y    N    4K     11.7k   46      11.7k   46      0%
 nvme       N    Y    N    64K    6453    403     8661    541    +34%
 nvme       N    Y    N    1M     649     649     1351    1351   +108%
 nvme       Y    N    Y    4K     372k    1456    433k    1693   +16%
 nvme       Y    N    Y    64K    33.0k   2064    74.7k   4669   +126%
 nvme       Y    N    Y    1M     2131    2131    5273    5273   +147%
 nvme       Y    Y    N    4K     56.7k   222     56.4k   220    -1%
 nvme       Y    Y    N    64K    13.4k   840     19.4k   1214   +45%
 nvme       Y    Y    N    1M     714     714     1504    1504   +111%

Thanks,
Yi.

Major changes since RFC v4:
 - Disable unsupported online defragmentation, do not fall back to
   buffer_head path.
 - Wite and wait data back while doing partial block truncate down to
   fix a stale data problem.
 - Disable the online changing of the inode journal flag to data=journal
   mode.
 - Since iomap can zero out dirty pages with unwritten extent, do not
   write data before zeroing out in ext4_zero_range(), and also do not
   zero partial blocks under a started journal handle.

[1] https://lore.kernel.org/linux-ext4/20241010133333.146793-1-yi.zhang@xxxxxxxxxx/

---
RFC v4: https://lore.kernel.org/linux-ext4/20240410142948.2817554-1-yi.zhang@xxxxxxxxxxxxxxx/
RFC v3: https://lore.kernel.org/linux-ext4/20240127015825.1608160-1-yi.zhang@xxxxxxxxxxxxxxx/
RFC v2: https://lore.kernel.org/linux-ext4/20240102123918.799062-1-yi.zhang@xxxxxxxxxxxxxxx/
RFC v1: https://lore.kernel.org/linux-ext4/20231123125121.4064694-1-yi.zhang@xxxxxxxxxxxxxxx/


Zhang Yi (27):
  ext4: remove writable userspace mappings before truncating page cache
  ext4: don't explicit update times in ext4_fallocate()
  ext4: don't write back data before punch hole in nojournal mode
  ext4: refactor ext4_punch_hole()
  ext4: refactor ext4_zero_range()
  ext4: refactor ext4_collapse_range()
  ext4: refactor ext4_insert_range()
  ext4: factor out ext4_do_fallocate()
  ext4: move out inode_lock into ext4_fallocate()
  ext4: move out common parts into ext4_fallocate()
  ext4: use reserved metadata blocks when splitting extent on endio
  ext4: introduce seq counter for the extent status entry
  ext4: add a new iomap aops for regular file's buffered IO path
  ext4: implement buffered read iomap path
  ext4: implement buffered write iomap path
  ext4: don't order data for inode with EXT4_STATE_BUFFERED_IOMAP
  ext4: implement writeback iomap path
  ext4: implement mmap iomap path
  ext4: do not always order data when partial zeroing out a block
  ext4: do not start handle if unnecessary while partial zeroing out a
    block
  ext4: implement zero_range iomap path
  ext4: disable online defrag when inode using iomap buffered I/O path
  ext4: disable inode journal mode when using iomap buffered I/O path
  ext4: partially enable iomap for the buffered I/O path of regular
    files
  ext4: enable large folio for regular file with iomap buffered I/O path
  ext4: change mount options code style
  ext4: introduce a mount option for iomap buffered I/O path

 fs/ext4/ext4.h              |  17 +-
 fs/ext4/ext4_jbd2.c         |   3 +-
 fs/ext4/ext4_jbd2.h         |   8 +
 fs/ext4/extents.c           | 568 +++++++++++----------------
 fs/ext4/extents_status.c    |  13 +-
 fs/ext4/file.c              |  19 +-
 fs/ext4/ialloc.c            |   5 +
 fs/ext4/inode.c             | 755 ++++++++++++++++++++++++++++++------
 fs/ext4/move_extent.c       |   7 +
 fs/ext4/page-io.c           | 105 +++++
 fs/ext4/super.c             | 185 ++++-----
 include/trace/events/ext4.h |  57 +--
 12 files changed, 1153 insertions(+), 589 deletions(-)

-- 
2.46.1





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