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[RFC,v3,00/26] ext4: use iomap for regular file's buffered IO path and enable large foilo

Message ID 20240127015825.1608160-1-yi.zhang@huaweicloud.com (mailing list archive)
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Series ext4: use iomap for regular file's buffered IO path and enable large foilo | expand

Message

Zhang Yi Jan. 27, 2024, 1:57 a.m. UTC
From: Zhang Yi <yi.zhang@huawei.com>

Hello,

This is the third version of RFC patch series that convert ext4 regular
file's buffered IO path to iomap and enable large folio. It's rebased on
6.7 and Christoph's "map multiple blocks per ->map_blocks in iomap
writeback" series [1]. I've fixed all issues found in the last about 3
weeks of stress tests and fault injection tests in v2. I hope I've
covered most of the corner cases, and any comments are welcome. :)

Changes since v2:
 - Update patch 1-6 to v3 [2].
 - iomap_zero and iomap_unshare don't need to update i_size and call
   iomap_write_failed(), introduce a new helper iomap_write_end_simple()
   to avoid doing that.
 - Factor out ext4_[ext|ind]_map_blocks() parts from ext4_map_blocks(),
   introduce a new helper ext4_iomap_map_one_extent() to allocate
   delalloc blocks in writeback, which is always under i_data_sem in
   write mode. This is done to prevent the writing back delalloc
   extents become stale if it raced by truncate.
 - Add a lock detection in mapping_clear_large_folios().
Changes since v1:
 - Introduce seq count for iomap buffered write and writeback to protect
   races from extents changes, e.g. truncate, mwrite.
 - Always allocate unwritten extents for new blocks, drop dioread_lock
   mode, and make no distinctions between dioread_lock and
   dioread_nolock.
 - Don't add ditry data range to jinode, drop data=ordered mode, and
   make no distinctions between data=ordered and data=writeback mode.
 - Postpone updating i_disksize to endio.
 - Allow splitting extents and use reserved space in endio.
 - Instead of reimplement a new delayed mapping helper
   ext4_iomap_da_map_blocks() for buffer write, try to reuse
   ext4_da_map_blocks().
 - Add support for disabling large folio on active inodes.
 - Support online defragmentation, make file fall back to buffer_head
   and disable large folio in ext4_move_extents().
 - Move ext4_nonda_switch() in advance to prevent deadlock in mwrite.
 - Add dirty_len and pos trace info to trace_iomap_writepage_map().
 - Update patch 1-6 to v2.

This series only support ext4 with the default features and mount
options, doesn't support inline_data, bigalloc, dax, fs_verity, fs_crypt
and data=journal mode, ext4 would fall back to buffer_head path
automatically if you enabled these features/options. Although it has
many limitations now, it can satisfy the requirements of common cases
and bring a great performance benefit.

Patch 1-6: this is a preparation series, it changes ext4_map_blocks()
and ext4_set_iomap() to recognize delayed only extents, I've send it out
separately [2].

Patch 7-8: these are two minor iomap changes, the first one is don't
update i_size and don't call iomap_write_failed() in zero_range, the
second one is for debug in iomap writeback path that I've discussed whit
Christoph [3].

Patch 9-15: this is another preparation series, including some changes
for delayed extents. Firstly, it factor out buffer_head from
ext4_da_map_blocks(), make it to support adding multi-blocks once a
time. Then make unwritten to written extents conversion in endio use to
reserved space, reduce the risk of potential data loss. Finally,
introduce a sequence counter for extent status tree, which is useful
for iomap buffer write and write back.

Patch 16-22: Implement buffered IO iomap path for read, write, mmap,
zero range, truncate and writeback, replace current buffered_head path.
Please look at the following patch for details.

Patch 23-26: Convert to iomap for regular file's buffered IO path
besides inline_data, bigalloc, dax, fs_verity, fs_crypt, and
data=journal mode, and enable large folio. It should be note that
buffered iomap path hasn't support Online defrag yet, so we need fall
back to buffer_head and disable large folio automatically if user call
EXT4_IOC_MOVE_EXT.

About Tests:
 - kvm-xfstests in auto mode, and about 3 weeks of stress tests and
   fault injection tests.
 - A performance tests below.

   Fio tests with psync on my machine with Intel Xeon Gold 6240 CPU
   with 400GB system ram, 200GB ramdisk and 1TB nvme ssd disk.

   == buffer read ==

                  buffer head        iomap with large folio
   type     bs    IOPS    BW(MiB/s)  IOPS    BW(MiB/s)
   ----------------------------------------------------
   hole     4K    565k    2206       811k    3167
   hole     64K   45.1k   2820       78.1k   4879
   hole     1M    2744    2744       4890    4891
   ramdisk  4K    436k    1703       554k    2163
   ramdisk  64K   29.6k   1848       44.0k   2747
   ramdisk  1M    1994    1995       2809    2809
   nvme     4K    306k    1196       324k    1267
   nvme     64K   19.3k   1208       24.3k   1517
   nvme     1M    1694    1694       2256    2256

   == buffer write ==

                                       buffer head    ext4_iomap    
   type   Overwrite Sync Writeback bs  IOPS   BW      IOPS   BW
   -------------------------------------------------------------
   cache    N       N    N         4K   395k   1544   415k   1621
   cache    N       N    N         64K  30.8k  1928   80.1k  5005
   cache    N       N    N         1M   1963   1963   5641   5642
   cache    Y       N    N         4K   423k   1652   443k   1730
   cache    Y       N    N         64K  33.0k  2063   80.8k  5051
   cache    Y       N    N         1M   2103   2103   5588   5589
   ramdisk  N       N    Y         4K   362k   1416   307k   1198
   ramdisk  N       N    Y         64K  22.4k  1399   64.8k  4050
   ramdisk  N       N    Y         1M   1670   1670   4559   4560
   ramdisk  N       Y    N         4K   9830   38.4   13.5k  52.8
   ramdisk  N       Y    N         64K  5834   365    10.1k  629
   ramdisk  N       Y    N         1M   1011   1011   2064   2064
   ramdisk  Y       N    Y         4K   397k   1550   409k   1598
   ramdisk  Y       N    Y         64K  29.2k  1827   73.6k  4597
   ramdisk  Y       N    Y         1M   1837   1837   4985   4985
   ramdisk  Y       Y    N         4K   173k   675    182k   710
   ramdisk  Y       Y    N         64K  17.7k  1109   33.7k  2105
   ramdisk  Y       Y    N         1M   1128   1129   1790   1791
   nvme     N       N    Y         4K   298k   1164   290k   1134
   nvme     N       N    Y         64K  21.5k  1343   57.4k  3590
   nvme     N       N    Y         1M   1308   1308   3664   3664
   nvme     N       Y    N         4K   10.7k  41.8   12.0k  46.9
   nvme     N       Y    N         64K  5962   373    8598   537
   nvme     N       Y    N         1M   676    677    1417   1418
   nvme     Y       N    Y         4K   366k   1430   373k   1456
   nvme     Y       N    Y         64K  26.7k  1670   56.8k  3547
   nvme     Y       N    Y         1M   1745   1746   3586   3586
   nvme     Y       Y    N         4K   59.0k  230    61.2k  239
   nvme     Y       Y    N         64K  13.0k  813    21.0k  1311
   nvme     Y       Y    N         1M   683    683    1368   1369
 
TODO
 - Keep on doing stress tests and fixing.
 - I will rebase and resend my another patch set "ext4: more accurate
   metadata reservaion for delalloc mount option[4]" later, it's useful
   for iomap conversion. After this series, I suppose we could totally
   drop ext4_nonda_switch() and prevent the risk of data loss caused by
   extents splitting.
 - Support for more features and mount options in the future.

[1] https://lore.kernel.org/linux-fsdevel/20231207072710.176093-1-hch@lst.de/
[2] https://lore.kernel.org/linux-ext4/20240105033018.1665752-1-yi.zhang@huaweicloud.com/
[3] https://lore.kernel.org/linux-fsdevel/20231207150311.GA18830@lst.de/
[4] https://lore.kernel.org/linux-ext4/20230824092619.1327976-1-yi.zhang@huaweicloud.com/

Thanks,
Yi.

---
v2: https://lore.kernel.org/linux-ext4/20240102123918.799062-1-yi.zhang@huaweicloud.com/
v1: https://lore.kernel.org/linux-ext4/20231123125121.4064694-1-yi.zhang@huaweicloud.com/

Zhang Yi (26):
  ext4: refactor ext4_da_map_blocks()
  ext4: convert to exclusive lock while inserting delalloc extents
  ext4: correct the hole length returned by ext4_map_blocks()
  ext4: add a hole extent entry in cache after punch
  ext4: make ext4_map_blocks() distinguish delalloc only extent
  ext4: make ext4_set_iomap() recognize IOMAP_DELALLOC map type
  iomap: don't increase i_size if it's not a write operation
  iomap: add pos and dirty_len into trace_iomap_writepage_map
  ext4: allow inserting delalloc extents with multi-blocks
  ext4: correct delalloc extent length
  ext4: also mark extent as delalloc if it's been unwritten
  ext4: factor out bh handles to ext4_da_get_block_prep()
  ext4: use reserved metadata blocks when splitting extent in endio
  ext4: factor out ext4_map_{create|query}_blocks()
  ext4: introduce seq counter for extent 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: implement writeback iomap path
  ext4: implement mmap iomap path
  ext4: implement zero_range iomap path
  ext4: writeback partial blocks before zero range
  ext4: fall back to buffer_head path for defrag
  ext4: partially enable iomap for regular file's buffered IO path
  filemap: support disable large folios on active inode
  ext4: enable large folio for regular file with iomap buffered IO path

 fs/ext4/ext4.h              |  14 +-
 fs/ext4/ext4_jbd2.c         |   6 +
 fs/ext4/ext4_jbd2.h         |   7 +
 fs/ext4/extents.c           | 149 +++---
 fs/ext4/extents_status.c    |  39 +-
 fs/ext4/extents_status.h    |   4 +-
 fs/ext4/file.c              |  19 +-
 fs/ext4/ialloc.c            |   5 +
 fs/ext4/inode.c             | 891 +++++++++++++++++++++++++++---------
 fs/ext4/move_extent.c       |  35 ++
 fs/ext4/page-io.c           | 107 +++++
 fs/ext4/super.c             |   3 +
 fs/iomap/buffered-io.c      |  30 +-
 fs/iomap/trace.h            |  43 +-
 include/linux/pagemap.h     |  14 +
 include/trace/events/ext4.h |  31 +-
 mm/readahead.c              |   6 +-
 17 files changed, 1109 insertions(+), 294 deletions(-)

Comments

Darrick J. Wong Feb. 12, 2024, 6:18 a.m. UTC | #1
On Sat, Jan 27, 2024 at 09:57:59AM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> Hello,
> 
> This is the third version of RFC patch series that convert ext4 regular
> file's buffered IO path to iomap and enable large folio. It's rebased on
> 6.7 and Christoph's "map multiple blocks per ->map_blocks in iomap
> writeback" series [1]. I've fixed all issues found in the last about 3
> weeks of stress tests and fault injection tests in v2. I hope I've
> covered most of the corner cases, and any comments are welcome. :)
> 
> Changes since v2:
>  - Update patch 1-6 to v3 [2].
>  - iomap_zero and iomap_unshare don't need to update i_size and call
>    iomap_write_failed(), introduce a new helper iomap_write_end_simple()
>    to avoid doing that.
>  - Factor out ext4_[ext|ind]_map_blocks() parts from ext4_map_blocks(),
>    introduce a new helper ext4_iomap_map_one_extent() to allocate
>    delalloc blocks in writeback, which is always under i_data_sem in
>    write mode. This is done to prevent the writing back delalloc
>    extents become stale if it raced by truncate.
>  - Add a lock detection in mapping_clear_large_folios().
> Changes since v1:
>  - Introduce seq count for iomap buffered write and writeback to protect
>    races from extents changes, e.g. truncate, mwrite.
>  - Always allocate unwritten extents for new blocks, drop dioread_lock
>    mode, and make no distinctions between dioread_lock and
>    dioread_nolock.
>  - Don't add ditry data range to jinode, drop data=ordered mode, and
>    make no distinctions between data=ordered and data=writeback mode.
>  - Postpone updating i_disksize to endio.
>  - Allow splitting extents and use reserved space in endio.
>  - Instead of reimplement a new delayed mapping helper
>    ext4_iomap_da_map_blocks() for buffer write, try to reuse
>    ext4_da_map_blocks().
>  - Add support for disabling large folio on active inodes.
>  - Support online defragmentation, make file fall back to buffer_head
>    and disable large folio in ext4_move_extents().
>  - Move ext4_nonda_switch() in advance to prevent deadlock in mwrite.
>  - Add dirty_len and pos trace info to trace_iomap_writepage_map().
>  - Update patch 1-6 to v2.
> 
> This series only support ext4 with the default features and mount
> options, doesn't support inline_data, bigalloc, dax, fs_verity, fs_crypt
> and data=journal mode, ext4 would fall back to buffer_head path

Do you plan to add bigalloc or !extents support as a part 2 patchset?

An ext2 port to iomap has been (vaguely) in the works for a while,
though iirc willy never got the performance to match because iomap
didn't have a mechanism for the caller to tell it "run the IO now even
though you don't have a complete page, because the indirect block is the
next block after the 11th block".

--D

> automatically if you enabled these features/options. Although it has
> many limitations now, it can satisfy the requirements of common cases
> and bring a great performance benefit.
> 
> Patch 1-6: this is a preparation series, it changes ext4_map_blocks()
> and ext4_set_iomap() to recognize delayed only extents, I've send it out
> separately [2].
> 
> Patch 7-8: these are two minor iomap changes, the first one is don't
> update i_size and don't call iomap_write_failed() in zero_range, the
> second one is for debug in iomap writeback path that I've discussed whit
> Christoph [3].
> 
> Patch 9-15: this is another preparation series, including some changes
> for delayed extents. Firstly, it factor out buffer_head from
> ext4_da_map_blocks(), make it to support adding multi-blocks once a
> time. Then make unwritten to written extents conversion in endio use to
> reserved space, reduce the risk of potential data loss. Finally,
> introduce a sequence counter for extent status tree, which is useful
> for iomap buffer write and write back.
> 
> Patch 16-22: Implement buffered IO iomap path for read, write, mmap,
> zero range, truncate and writeback, replace current buffered_head path.
> Please look at the following patch for details.
> 
> Patch 23-26: Convert to iomap for regular file's buffered IO path
> besides inline_data, bigalloc, dax, fs_verity, fs_crypt, and
> data=journal mode, and enable large folio. It should be note that
> buffered iomap path hasn't support Online defrag yet, so we need fall
> back to buffer_head and disable large folio automatically if user call
> EXT4_IOC_MOVE_EXT.
> 
> About Tests:
>  - kvm-xfstests in auto mode, and about 3 weeks of stress tests and
>    fault injection tests.
>  - A performance tests below.
> 
>    Fio tests with psync on my machine with Intel Xeon Gold 6240 CPU
>    with 400GB system ram, 200GB ramdisk and 1TB nvme ssd disk.
> 
>    == buffer read ==
> 
>                   buffer head        iomap with large folio
>    type     bs    IOPS    BW(MiB/s)  IOPS    BW(MiB/s)
>    ----------------------------------------------------
>    hole     4K    565k    2206       811k    3167
>    hole     64K   45.1k   2820       78.1k   4879
>    hole     1M    2744    2744       4890    4891
>    ramdisk  4K    436k    1703       554k    2163
>    ramdisk  64K   29.6k   1848       44.0k   2747
>    ramdisk  1M    1994    1995       2809    2809
>    nvme     4K    306k    1196       324k    1267
>    nvme     64K   19.3k   1208       24.3k   1517
>    nvme     1M    1694    1694       2256    2256
> 
>    == buffer write ==
> 
>                                        buffer head    ext4_iomap    
>    type   Overwrite Sync Writeback bs  IOPS   BW      IOPS   BW
>    -------------------------------------------------------------
>    cache    N       N    N         4K   395k   1544   415k   1621
>    cache    N       N    N         64K  30.8k  1928   80.1k  5005
>    cache    N       N    N         1M   1963   1963   5641   5642
>    cache    Y       N    N         4K   423k   1652   443k   1730
>    cache    Y       N    N         64K  33.0k  2063   80.8k  5051
>    cache    Y       N    N         1M   2103   2103   5588   5589
>    ramdisk  N       N    Y         4K   362k   1416   307k   1198
>    ramdisk  N       N    Y         64K  22.4k  1399   64.8k  4050
>    ramdisk  N       N    Y         1M   1670   1670   4559   4560
>    ramdisk  N       Y    N         4K   9830   38.4   13.5k  52.8
>    ramdisk  N       Y    N         64K  5834   365    10.1k  629
>    ramdisk  N       Y    N         1M   1011   1011   2064   2064
>    ramdisk  Y       N    Y         4K   397k   1550   409k   1598
>    ramdisk  Y       N    Y         64K  29.2k  1827   73.6k  4597
>    ramdisk  Y       N    Y         1M   1837   1837   4985   4985
>    ramdisk  Y       Y    N         4K   173k   675    182k   710
>    ramdisk  Y       Y    N         64K  17.7k  1109   33.7k  2105
>    ramdisk  Y       Y    N         1M   1128   1129   1790   1791
>    nvme     N       N    Y         4K   298k   1164   290k   1134
>    nvme     N       N    Y         64K  21.5k  1343   57.4k  3590
>    nvme     N       N    Y         1M   1308   1308   3664   3664
>    nvme     N       Y    N         4K   10.7k  41.8   12.0k  46.9
>    nvme     N       Y    N         64K  5962   373    8598   537
>    nvme     N       Y    N         1M   676    677    1417   1418
>    nvme     Y       N    Y         4K   366k   1430   373k   1456
>    nvme     Y       N    Y         64K  26.7k  1670   56.8k  3547
>    nvme     Y       N    Y         1M   1745   1746   3586   3586
>    nvme     Y       Y    N         4K   59.0k  230    61.2k  239
>    nvme     Y       Y    N         64K  13.0k  813    21.0k  1311
>    nvme     Y       Y    N         1M   683    683    1368   1369
>  
> TODO
>  - Keep on doing stress tests and fixing.
>  - I will rebase and resend my another patch set "ext4: more accurate
>    metadata reservaion for delalloc mount option[4]" later, it's useful
>    for iomap conversion. After this series, I suppose we could totally
>    drop ext4_nonda_switch() and prevent the risk of data loss caused by
>    extents splitting.
>  - Support for more features and mount options in the future.
> 
> [1] https://lore.kernel.org/linux-fsdevel/20231207072710.176093-1-hch@lst.de/
> [2] https://lore.kernel.org/linux-ext4/20240105033018.1665752-1-yi.zhang@huaweicloud.com/
> [3] https://lore.kernel.org/linux-fsdevel/20231207150311.GA18830@lst.de/
> [4] https://lore.kernel.org/linux-ext4/20230824092619.1327976-1-yi.zhang@huaweicloud.com/
> 
> Thanks,
> Yi.
> 
> ---
> v2: https://lore.kernel.org/linux-ext4/20240102123918.799062-1-yi.zhang@huaweicloud.com/
> v1: https://lore.kernel.org/linux-ext4/20231123125121.4064694-1-yi.zhang@huaweicloud.com/
> 
> Zhang Yi (26):
>   ext4: refactor ext4_da_map_blocks()
>   ext4: convert to exclusive lock while inserting delalloc extents
>   ext4: correct the hole length returned by ext4_map_blocks()
>   ext4: add a hole extent entry in cache after punch
>   ext4: make ext4_map_blocks() distinguish delalloc only extent
>   ext4: make ext4_set_iomap() recognize IOMAP_DELALLOC map type
>   iomap: don't increase i_size if it's not a write operation
>   iomap: add pos and dirty_len into trace_iomap_writepage_map
>   ext4: allow inserting delalloc extents with multi-blocks
>   ext4: correct delalloc extent length
>   ext4: also mark extent as delalloc if it's been unwritten
>   ext4: factor out bh handles to ext4_da_get_block_prep()
>   ext4: use reserved metadata blocks when splitting extent in endio
>   ext4: factor out ext4_map_{create|query}_blocks()
>   ext4: introduce seq counter for extent 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: implement writeback iomap path
>   ext4: implement mmap iomap path
>   ext4: implement zero_range iomap path
>   ext4: writeback partial blocks before zero range
>   ext4: fall back to buffer_head path for defrag
>   ext4: partially enable iomap for regular file's buffered IO path
>   filemap: support disable large folios on active inode
>   ext4: enable large folio for regular file with iomap buffered IO path
> 
>  fs/ext4/ext4.h              |  14 +-
>  fs/ext4/ext4_jbd2.c         |   6 +
>  fs/ext4/ext4_jbd2.h         |   7 +
>  fs/ext4/extents.c           | 149 +++---
>  fs/ext4/extents_status.c    |  39 +-
>  fs/ext4/extents_status.h    |   4 +-
>  fs/ext4/file.c              |  19 +-
>  fs/ext4/ialloc.c            |   5 +
>  fs/ext4/inode.c             | 891 +++++++++++++++++++++++++++---------
>  fs/ext4/move_extent.c       |  35 ++
>  fs/ext4/page-io.c           | 107 +++++
>  fs/ext4/super.c             |   3 +
>  fs/iomap/buffered-io.c      |  30 +-
>  fs/iomap/trace.h            |  43 +-
>  include/linux/pagemap.h     |  14 +
>  include/trace/events/ext4.h |  31 +-
>  mm/readahead.c              |   6 +-
>  17 files changed, 1109 insertions(+), 294 deletions(-)
> 
> -- 
> 2.39.2
> 
>
Ritesh Harjani (IBM) Feb. 12, 2024, 9:16 a.m. UTC | #2
"Darrick J. Wong" <djwong@kernel.org> writes:

> On Sat, Jan 27, 2024 at 09:57:59AM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>> 
>> Hello,
>> 
>> This is the third version of RFC patch series that convert ext4 regular
>> file's buffered IO path to iomap and enable large folio. It's rebased on
>> 6.7 and Christoph's "map multiple blocks per ->map_blocks in iomap
>> writeback" series [1]. I've fixed all issues found in the last about 3
>> weeks of stress tests and fault injection tests in v2. I hope I've
>> covered most of the corner cases, and any comments are welcome. :)
>> 
>> Changes since v2:
>>  - Update patch 1-6 to v3 [2].
>>  - iomap_zero and iomap_unshare don't need to update i_size and call
>>    iomap_write_failed(), introduce a new helper iomap_write_end_simple()
>>    to avoid doing that.
>>  - Factor out ext4_[ext|ind]_map_blocks() parts from ext4_map_blocks(),
>>    introduce a new helper ext4_iomap_map_one_extent() to allocate
>>    delalloc blocks in writeback, which is always under i_data_sem in
>>    write mode. This is done to prevent the writing back delalloc
>>    extents become stale if it raced by truncate.
>>  - Add a lock detection in mapping_clear_large_folios().
>> Changes since v1:
>>  - Introduce seq count for iomap buffered write and writeback to protect
>>    races from extents changes, e.g. truncate, mwrite.
>>  - Always allocate unwritten extents for new blocks, drop dioread_lock
>>    mode, and make no distinctions between dioread_lock and
>>    dioread_nolock.
>>  - Don't add ditry data range to jinode, drop data=ordered mode, and
>>    make no distinctions between data=ordered and data=writeback mode.
>>  - Postpone updating i_disksize to endio.
>>  - Allow splitting extents and use reserved space in endio.
>>  - Instead of reimplement a new delayed mapping helper
>>    ext4_iomap_da_map_blocks() for buffer write, try to reuse
>>    ext4_da_map_blocks().
>>  - Add support for disabling large folio on active inodes.
>>  - Support online defragmentation, make file fall back to buffer_head
>>    and disable large folio in ext4_move_extents().
>>  - Move ext4_nonda_switch() in advance to prevent deadlock in mwrite.
>>  - Add dirty_len and pos trace info to trace_iomap_writepage_map().
>>  - Update patch 1-6 to v2.
>> 
>> This series only support ext4 with the default features and mount
>> options, doesn't support inline_data, bigalloc, dax, fs_verity, fs_crypt
>> and data=journal mode, ext4 would fall back to buffer_head path
>
> Do you plan to add bigalloc or !extents support as a part 2 patchset?
>

Hi Darrick,

> An ext2 port to iomap has been (vaguely) in the works for a while,

yes, we have [1][2]. I am in the process of rebasing that work on the latest
upstream. It's been a while since my last post since I have been pulled
into some other internal work, sorry about that.

> though iirc willy never got the performance to match because iomap

Ohh, can you help me provide details on what performance benchmark was
run? I can try and run them when I rebase.

> didn't have a mechanism for the caller to tell it "run the IO now even
> though you don't have a complete page, because the indirect block is the
> next block after the 11th block".

Do you mean this for a large folio? I still didn't get the problem you
are referring here. Can you please help me explain why could that be a
problem?

[1]: https://lore.kernel.org/linux-ext4/9cdd449fc1d63cf2dba17cfa2fa7fb29b8f96a46.1700506526.git.ritesh.list@gmail.com/
[2]: https://lore.kernel.org/linux-ext4/8734wnj53k.fsf@doe.com/

-ritesh
Matthew Wilcox Feb. 12, 2024, 10:24 a.m. UTC | #3
On Mon, Feb 12, 2024 at 02:46:10PM +0530, Ritesh Harjani wrote:
> "Darrick J. Wong" <djwong@kernel.org> writes:
> > though iirc willy never got the performance to match because iomap
> 
> Ohh, can you help me provide details on what performance benchmark was
> run? I can try and run them when I rebase.

I didn't run a benchmark, we just knew what would happen (on rotating
storage anyway).

> > didn't have a mechanism for the caller to tell it "run the IO now even
> > though you don't have a complete page, because the indirect block is the
> > next block after the 11th block".
> 
> Do you mean this for a large folio? I still didn't get the problem you
> are referring here. Can you please help me explain why could that be a
> problem?

A classic ext2 filesystem lays out a 16kB file like this (with 512
byte blocks):

file offset	disk block
0-6KiB		1000-1011
6KiB-16KiB	1013-1032

What's in block 1012?  The indirect block!  The block which tells ext2
that blocks 12-31 of the file are in disk blocks 1013-1032.  So we can't
issue the read for them until we've finished the read for block 1012.

Buffer heads have a solution for this, BH_Boundary.  ext2 sets it for
block 11 which prompts mpage.c to submit the read immediately (see
the various calls to buffer_boundary()).  Then ext2 will submit the read
for block 1012 and the two reads will be coalesced by the IO scheduler.
So we still end up doing two reads instead of one, but that's
unavoidable because fragmentation might have meant that 6KiB-16KiB were
not stored at 1013-1032.

There's no equivalent iomap solution.  What needs to happen is:

 - iomap_folio_state->read_bytes_pending needs to be initialised to
   folio_size(), not 0.
 - Remove "ifs->read_bytes_pending += plen" from iomap_readpage_iter()
 - Subtract plen in the iomap_block_needs_zeroing() case
 - Submit a bio at the end of each iomap_readpage_iter() call

Now iomap will behave the same way as mpage, only without needing a
flag to do it (instead it will assume that the filesystem coalesces
adjacent ranges, which it should do anyway for good performance).
Zhang Yi Feb. 17, 2024, 9:31 a.m. UTC | #4
On 2024/2/12 14:18, Darrick J. Wong wrote:
> On Sat, Jan 27, 2024 at 09:57:59AM +0800, Zhang Yi wrote:
>> From: Zhang Yi <yi.zhang@huawei.com>
>>
>> Hello,
>>
>> This is the third version of RFC patch series that convert ext4 regular
>> file's buffered IO path to iomap and enable large folio. It's rebased on
>> 6.7 and Christoph's "map multiple blocks per ->map_blocks in iomap
>> writeback" series [1]. I've fixed all issues found in the last about 3
>> weeks of stress tests and fault injection tests in v2. I hope I've
>> covered most of the corner cases, and any comments are welcome. :)
>>
>> Changes since v2:
>>  - Update patch 1-6 to v3 [2].
>>  - iomap_zero and iomap_unshare don't need to update i_size and call
>>    iomap_write_failed(), introduce a new helper iomap_write_end_simple()
>>    to avoid doing that.
>>  - Factor out ext4_[ext|ind]_map_blocks() parts from ext4_map_blocks(),
>>    introduce a new helper ext4_iomap_map_one_extent() to allocate
>>    delalloc blocks in writeback, which is always under i_data_sem in
>>    write mode. This is done to prevent the writing back delalloc
>>    extents become stale if it raced by truncate.
>>  - Add a lock detection in mapping_clear_large_folios().
>> Changes since v1:
>>  - Introduce seq count for iomap buffered write and writeback to protect
>>    races from extents changes, e.g. truncate, mwrite.
>>  - Always allocate unwritten extents for new blocks, drop dioread_lock
>>    mode, and make no distinctions between dioread_lock and
>>    dioread_nolock.
>>  - Don't add ditry data range to jinode, drop data=ordered mode, and
>>    make no distinctions between data=ordered and data=writeback mode.
>>  - Postpone updating i_disksize to endio.
>>  - Allow splitting extents and use reserved space in endio.
>>  - Instead of reimplement a new delayed mapping helper
>>    ext4_iomap_da_map_blocks() for buffer write, try to reuse
>>    ext4_da_map_blocks().
>>  - Add support for disabling large folio on active inodes.
>>  - Support online defragmentation, make file fall back to buffer_head
>>    and disable large folio in ext4_move_extents().
>>  - Move ext4_nonda_switch() in advance to prevent deadlock in mwrite.
>>  - Add dirty_len and pos trace info to trace_iomap_writepage_map().
>>  - Update patch 1-6 to v2.
>>
>> This series only support ext4 with the default features and mount
>> options, doesn't support inline_data, bigalloc, dax, fs_verity, fs_crypt
>> and data=journal mode, ext4 would fall back to buffer_head path
> 
> Do you plan to add bigalloc or !extents support as a part 2 patchset?

Hello,

Sorry for the late reply since I was on the vacation of Chinese New Year.
I've been working on bigalloc support recently and it's going relatively
well, but have no plans to support !extents yet, I would start looking
into it after I finish rebasing my another patch set "ext4: more
accurate metadata reservaion for delalloc mount option" mentioned in my
TODO list.

> 
> An ext2 port to iomap has been (vaguely) in the works for a while,
> though iirc willy never got the performance to match because iomap
> didn't have a mechanism for the caller to tell it "run the IO now even
> though you don't have a complete page, because the indirect block is the
> next block after the 11th block".
> 

Thanks for pointing this out and the explanation given by Matthew. IIUC,
this problem also affects ext4 in !extents mode, but not affects bigalloc,
right?

Thanks,
Yi.