linux-ext4 - [PATCH V4 0/5] ext4: get discard out of jbd2 commit kthread

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Date:   Mon, 30 Aug 2021 15:52:41 +0800
From:   Wang Jianchao <jianchao.wan9@...il.com>
To:     tytso@....edu, adilger.kernel@...ger.ca
Cc:     jack@...e.cz, guoqing.jiang@...ux.dev,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: [PATCH V4 0/5] ext4: get discard out of jbd2 commit kthread

Hi all

This is the version 4 patch set that attempts to get discard out of the jbd2
commit kthread. When the user delete a lot data and cause discard flooding,
the jbd2 commit kthread can be blocked for very long time and then all of
the metadata operations are blocked due to no journal space.

The xfstest with following parameters,
MODULAR=0
TEST_DIR=/mnt/test
TEST_DEV=/dev/nbd37p1
SCRATCH_MNT=/mnt/scratch
SCRATCH_DEV=/dev/nbd37p2
MOUNT_OPTIONS="-o discard"
has passed. The result is consistent w/ or w/o this patch set.

There are 5 patches,

Patch 1 ~ 3, there are no functional changes in them, but just some preparation
for following patches

Patch 4 introduces a async kworker to do discard in fstrim fation which implements
the core idea of this patch set.

Patch 5 flush discard background work in ext4_should_retry_alloc, This fix the generic/371

Any comments are welcome ;)

V3 -> V4:
 - In patch 1, avoid modify two lines in patch 1 when remove 'grou'p parameter
 - In patch 4, remove redunbant queue_work() in ext4_mb_release(). And queue background
   discard work to system_unbound_wq as it is not a urgent task.
 - In patch 5, flush discard kwork in ext4_should_retry_alloc(), instead of invoke
   ext4_should_retry_alloc in fallocate again.

V2 -> V3
 - Get rid of the per block group rb tree which carries freed entry. It is not neccesary
   because we have done aggregation when wait for journal commit. Just use a list
   to carry the free entries.

V1 -> V2
 - free the blocks back to mb buddy after commit and then do ftrim fashion discard

 fs/ext4/balloc.c  |   8 ++-
 fs/ext4/ext4.h    |   3 ++
 fs/ext4/mballoc.c | 216 +++++++++++++++++++++++++++++++++++++++++++++++++++-----------------------------
 3 files changed, 148 insertions(+), 79 deletions(-)