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Jan Kara 9339cf3876 writeback: Avoid excessively long inode switching times 
[ Upstream commit 9a6ebbdbd4 ]

With lazytime mount option enabled we can be switching many dirty inodes
on cgroup exit to the parent cgroup. The numbers observed in practice
when systemd slice of a large cron job exits can easily reach hundreds
of thousands or millions. The logic in inode_do_switch_wbs() which sorts
the inode into appropriate place in b_dirty list of the target wb
however has linear complexity in the number of dirty inodes thus overall
time complexity of switching all the inodes is quadratic leading to
workers being pegged for hours consuming 100% of the CPU and switching
inodes to the parent wb.

Simple reproducer of the issue:
  FILES=10000
  # Filesystem mounted with lazytime mount option
  MNT=/mnt/
  echo "Creating files and switching timestamps"
  for (( j = 0; j < 50; j ++ )); do
      mkdir $MNT/dir$j
      for (( i = 0; i < $FILES; i++ )); do
          echo "foo" >$MNT/dir$j/file$i
      done
      touch -a -t 202501010000 $MNT/dir$j/file*
  done
  wait
  echo "Syncing and flushing"
  sync
  echo 3 >/proc/sys/vm/drop_caches

  echo "Reading all files from a cgroup"
  mkdir /sys/fs/cgroup/unified/mycg1 || exit
  echo $$ >/sys/fs/cgroup/unified/mycg1/cgroup.procs || exit
  for (( j = 0; j < 50; j ++ )); do
      cat /mnt/dir$j/file* >/dev/null &
  done
  wait
  echo "Switching wbs"
  # Now rmdir the cgroup after the script exits

We need to maintain b_dirty list ordering to keep writeback happy so
instead of sorting inode into appropriate place just append it at the
end of the list and clobber dirtied_time_when. This may result in inode
writeback starting later after cgroup switch however cgroup switches are
rare so it shouldn't matter much. Since the cgroup had write access to
the inode, there are no practical concerns of the possible DoS issues.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-10-19 16:21:54 +02:00
arch minmax: make generic MIN() and MAX() macros available everywhere 2025-10-19 16:21:52 +02:00
block block: use int to store blk_stack_limits() return value 2025-10-19 16:21:30 +02:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:22:01 +02:00
crypto crypto: essiv - Check ssize for decryption and in-place encryption 2025-10-19 16:21:43 +02:00
Documentation fs: Add 'initramfs_options' to set initramfs mount options 2025-10-19 16:21:54 +02:00
drivers minmax: don't use max() in situations that want a C constant expression 2025-10-19 16:21:52 +02:00
fs writeback: Avoid excessively long inode switching times 2025-10-19 16:21:54 +02:00
include minmax.h: remove some #defines that are only expanded once 2025-10-19 16:21:54 +02:00
init init: handle bootloader identifier in kernel parameters 2025-10-19 16:21:45 +02:00
io_uring io_uring: fix possible deadlock in io_register_iowq_max_workers() 2024-11-17 15:06:25 +01:00
ipc ipc: fix to protect IPCS lookups using RCU 2025-06-27 11:05:26 +01:00
kernel pid: Add a judgment for ns null in pid_nr_ns 2025-10-19 16:21:54 +02:00
lib minmax: don't use max() in situations that want a C constant expression 2025-10-19 16:21:52 +02:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm minmax: make generic MIN() and MAX() macros available everywhere 2025-10-19 16:21:52 +02:00
net minmax: add a few more MIN_T/MAX_T users 2025-10-19 16:21:52 +02:00
samples samples: mei: Fix building on musl libc 2025-08-28 16:24:07 +02:00
scripts locking: Introduce __cleanup() based infrastructure 2025-10-19 16:21:51 +02:00
security KEYS: trusted_tpm1: Compare HMAC values in constant time 2025-10-19 16:21:45 +02:00
sound ASoC: wcd934x: fix error handling in wcd934x_codec_parse_data() 2025-10-19 16:21:50 +02:00
tools minmax: make generic MIN() and MAX() macros available everywhere 2025-10-19 16:21:52 +02:00
usr kbuild: hdrcheck: fix cross build with clang 2025-06-27 11:05:22 +01:00
virt KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin() 2024-10-22 15:40:41 +02:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Remove *.orig pattern from .gitignore 2024-10-17 15:11:10 +02:00
.mailmap mailmap: add Andrej Shadura 2021-10-18 20:22:03 -10:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS trace: Relocate event helper files 2024-04-10 16:19:24 +02:00
Makefile Linux 5.15.194 2025-10-02 13:39:15 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.