[PATCH v8 0/8] cgroup, blkcg: prevent dirty inodes to pin dying memory cgroups

Roman Gushchin
2021-06-08T01:31:15Z

When an inode is getting dirty for the first time it's associated
with a wb structure (see __inode_attach_wb()). It can later be
switched to another wb (if e.g. some other cgroup is writing a lot of
data to the same inode), but otherwise stays attached to the original
wb until being reclaimed.

The problem is that the wb structure holds a reference to the original
memory and blkcg cgroups. So if an inode has been dirty once and later
is actively used in read-only mode, it has a good chance to pin down
the original memory and blkcg cgroups forewer. This is often the case with
services bringing data for other services, e.g. updating some rpm
packages.

In the real life it becomes a problem due to a large size of the memcg
structure, which can easily be 1000x larger than an inode. Also a
really large number of dying cgroups can raise different scalability
issues, e.g. making the memory reclaim costly and less effective.

To solve the problem inodes should be eventually detached from the
corresponding writeback structure. It's inefficient to do it after
every writeback completion. Instead it can be done whenever the
original memory cgroup is offlined and writeback structure is getting
killed. Scanning over a (potentially long) list of inodes and detach
them from the writeback structure can take quite some time. To avoid
scanning all inodes, attached inodes are kept on a new list (b_attached).
To make it less noticeable to a user, the scanning and switching is performed
from a work context.

Big thanks to Jan Kara, Dennis Zhou, Hillf Danton and Tejun Heo for their ideas
and contribution to this patchset.

v8:
- switch inodes to a nearest living ancestor wb instead of root wb
- added two inodes switching fixes suggested by Jan Kara

v7:
- shared locking for multiple inode switching
- introduced inode_prepare_wbs_switch() helper
- extended the pre-switch inode check for I_WILL_FREE
- added comments here and there

v6:
- extended and reused wbs switching functionality to switch inodes
on cgwb cleanup
- fixed offline_list handling
- switched to the unbound_wq
- other minor fixes

v5:
- switch inodes to bdi->wb instead of zeroing inode->i_wb
- split the single patch into two
- only cgwbs maintain lists of attached inodes
- added cond_resched()
- fixed !CONFIG_CGROUP_WRITEBACK handling
- extended list of prohibited inodes flag
- other small fixes

Roman Gushchin (8):
writeback, cgroup: do not switch inodes with I_WILL_FREE flag
writeback, cgroup: add smp_mb() to cgroup_writeback_umount()
writeback, cgroup: increment isw_nr_in_flight before grabbing an inode
writeback, cgroup: switch to rcu_work API in inode_switch_wbs()
writeback, cgroup: keep list of inodes attached to bdi_writeback
writeback, cgroup: split out the functional part of
inode_switch_wbs_work_fn()
writeback, cgroup: support switching multiple inodes at once
writeback, cgroup: release dying cgwbs by switching attached inodes

fs/fs-writeback.c | 323 +++++++++++++++++++++----------
include/linux/backing-dev-defs.h | 20 +-
include/linux/writeback.h | 1 +
mm/backing-dev.c | 69 ++++++-
4 files changed, 312 insertions(+), 101 deletions(-)

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2.31.1