linux-ext4 - Re: [PATCH v4 00/11] ext4: fix insufficient credits when writing back large folios

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Message-ID: <CA+G9fYvXTCK5PHSazWkE6yww1QJA=wwb+1xV0udMSXgc63P_6A@mail.gmail.com>
Date: Wed, 9 Jul 2025 13:23:25 +0530
From: Naresh Kamboju <naresh.kamboju@...aro.org>
To: Zhang Yi <yi.zhang@...weicloud.com>
Cc: linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, tytso@....edu, adilger.kernel@...ger.ca, 
	jack@...e.cz, ojaswin@...ux.ibm.com, sashal@...nel.org, jiangqi903@...il.com, 
	yi.zhang@...wei.com, libaokun1@...wei.com, yukuai3@...wei.com, 
	yangerkun@...wei.com
Subject: Re: [PATCH v4 00/11] ext4: fix insufficient credits when writing back
 large folios

On Mon, 7 Jul 2025 at 19:53, Zhang Yi <yi.zhang@...weicloud.com> wrote:
>
> From: Zhang Yi <yi.zhang@...wei.com>
>
> Changes since v3:
>  - Fix the end_pos assignment in patch 01.
>  - Rename mpage_submit_buffers() to mpage_submit_partial_folio(), and
>    fix a left shift out-of-bounds problem in patch 03.
>  - Fix the spelling errors in patch 04.
>  - Add a comment for NULL 'handle' test in
>    ext4_journal_ensure_extent_credits().
>  - Add patch 11 to limit the maximum order of the folio to 2048 fs
>    blocks, prevent the overestimation of reserve journal credits during
>    folios write-back.
> Changes since v2:
>  - Convert the processing of folios writeback in bytes instead of pages.
>  - Refactor ext4_page_mkwrite() and ensure journal credits in
>    ext4_block_write_begin() instead of in _ext4_get_block().
>  - Enhance tracepoints in ext4_do_writepages().
>  - Replace the outdated ext4_da_writepages_trans_blocks() and
>    ext4_writepage_trans_blocks() with the new helper used to reserve
>    credits for a single extent.
> Changes since v1:
>  - Make the write-back process supports writing a partial folio if it
>    exits the mapping loop prematurely due to insufficient sapce or
>    journal credits, it also fix the potential stale data and
>    inconsistency issues.
>  - Fix the same issue regarding the allocation of blocks in
>    ext4_write_begin() and ext4_page_mkwrite() when delalloc is not
>    enabled.
>
> v3: https://lore.kernel.org/linux-ext4/20250701130635.4079595-1-yi.zhang@huaweicloud.com/
> v2: https://lore.kernel.org/linux-ext4/20250611111625.1668035-1-yi.zhang@huaweicloud.com/
> v1: https://lore.kernel.org/linux-ext4/20250530062858.458039-1-yi.zhang@huaweicloud.com/
>
> Original Description
>
> This series addresses the issue that Jan pointed out regarding large
> folios support for ext4[1]. The problem is that the credits calculation
> may insufficient in ext4_meta_trans_blocks() when allocating blocks
> during write back a sufficiently large and discontinuous folio, it
> doesn't involve the credits for updating bitmap and group descriptor
> block. However, if we fix this issue, it may lead to significant
> overestimation on the some filesystems with a lot of block groups.
>
> The solution involves first ensure that the current journal transaction
> has enough credits when we mapping an extent during allocating blocks.
> Then if the credits reach the upper limit, exit the current mapping
> loop, submit the partial folio and restart a new transaction. Finally,
> fix the wrong credits calculation in ext4_meta_trans_blocks(). Please
> see the following patches for details.

I have applied this patch set on top of the Linux next tree and performed
testing. The previously reported regressions [a] are no longer observed.
Thank you for providing the fix.

Tested-by: Linux Kernel Functional Testing <lkft@...aro.org>

Reference link:
[a] https://lore.kernel.org/all/CA+G9fYsyYQ3ZL4xaSg1-Tt5Evto7Zd+hgNWZEa9cQLbahA1+xg@mail.gmail.com/

>
> [1] https://lore.kernel.org/linux-ext4/ht54j6bvjmiqt62xmcveqlo7bmrunqs4ji7wikfteftdjijzek@7tz5gpejaoen/
>
> Thanks,
> Yi.
>
> Zhang Yi (11):
>   ext4: process folios writeback in bytes
>   ext4: move the calculation of wbc->nr_to_write to mpage_folio_done()
>   ext4: fix stale data if it bail out of the extents mapping loop
>   ext4: refactor the block allocation process of ext4_page_mkwrite()
>   ext4: restart handle if credits are insufficient during allocating
>     blocks
>   ext4: enhance tracepoints during the folios writeback
>   ext4: correct the reserved credits for extent conversion
>   ext4: reserved credits for one extent during the folio writeback
>   ext4: replace ext4_writepage_trans_blocks()
>   ext4: fix insufficient credits calculation in ext4_meta_trans_blocks()
>   ext4: limit the maximum folio order
>
>  fs/ext4/ext4.h              |   4 +-
>  fs/ext4/extents.c           |   6 +-
>  fs/ext4/ialloc.c            |   3 +-
>  fs/ext4/inline.c            |   6 +-
>  fs/ext4/inode.c             | 349 +++++++++++++++++++++++-------------
>  fs/ext4/move_extent.c       |   3 +-
>  fs/ext4/xattr.c             |   2 +-
>  include/trace/events/ext4.h |  47 ++++-
>  8 files changed, 272 insertions(+), 148 deletions(-)
>
> --
> 2.46.1

--
Linaro LKFT
https://lkft.linaro.org