Hello, On Thu, Jun 03, 2021 at 06:31:53PM -0700, Roman Gushchin wrote: > 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 and Hillf Danton for their ideas and > contribution to this patchset. > > 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 (6): > writeback, cgroup: do not switch inodes with I_WILL_FREE flag > 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 | 302 +++++++++++++++++++++---------- > include/linux/backing-dev-defs.h | 20 +- > include/linux/writeback.h | 1 + > mm/backing-dev.c | 69 ++++++- > 4 files changed, 293 insertions(+), 99 deletions(-) > > -- > 2.31.1 > I too am a bit late to the party. Feel free to add mine as well to the series. Acked-by: Dennis Zhou <dennis@xxxxxxxxxx> I left my one comment on the last patch regarding a possible future extension. Thanks, Dennis
