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 is performed from a work context. Big thanks to Jan Kara and Dennis Zhou for their ideas and contribution to the previous iterations of this patch. 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 (2): writeback, cgroup: keep list of inodes attached to bdi_writeback writeback, cgroup: release dying cgwbs by switching attached inodes fs/fs-writeback.c | 101 +++++++++++++++++++++++++------ include/linux/backing-dev-defs.h | 2 + include/linux/backing-dev.h | 7 +++ include/linux/writeback.h | 2 + mm/backing-dev.c | 63 ++++++++++++++++++- 5 files changed, 156 insertions(+), 19 deletions(-) -- 2.31.1
