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    Subject[PATCH v7 0/6] cgroup, blkcg: prevent dirty inodes to pin dying memory cgroups
    Date
    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

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