[PATCHSET] writeback, memcg: Implement foreign inode flushing

Tejun Heo
2019-08-03T14:02:04Z

Hello,

There's an inherent mismatch between memcg and writeback. The former
trackes ownership per-page while the latter per-inode. This was a
deliberate design decision because honoring per-page ownership in the
writeback path is complicated, may lead to higher CPU and IO overheads
and deemed unnecessary given that write-sharing an inode across
different cgroups isn't a common use-case.

Combined with inode majority-writer ownership switching, this works
well enough in most cases but there are some pathological cases. For
example, let's say there are two cgroups A and B which keep writing to
different but confined parts of the same inode. B owns the inode and
A's memory is limited far below B's. A's dirty ratio can rise enough
to trigger balance_dirty_pages() sleeps but B's can be low enough to
avoid triggering background writeback. A will be slowed down without
a way to make writeback of the dirty pages happen.

This patchset implements foreign dirty recording and foreign mechanism
so that when a memcg encounters a condition as above it can trigger
flushes on bdi_writebacks which can clean its pages. Please see the
last patch for more details.

This patchset contains the following four patches.

0001-writeback-Generalize-and-expose-wb_completion.patch
0002-bdi-Add-bdi-id.patch
0003-writeback-memcg-Implement-cgroup_writeback_by_id.patch
0004-writeback-memcg-Implement-foreign-dirty-flushing.patch

0001-0003 are prep patches which expose wb_completion and implement
bdi->id and flushing by bdi and memcg IDs.

0004 implement foreign inode flushing.

Thanks. diffstat follows.

fs/fs-writeback.c | 111 ++++++++++++++++++++++++----------
include/linux/backing-dev-defs.h | 23 +++++++
include/linux/backing-dev.h | 3
include/linux/memcontrol.h | 35 ++++++++++
include/linux/writeback.h | 4 +
mm/backing-dev.c | 65 +++++++++++++++++++-
mm/memcontrol.c | 125 +++++++++++++++++++++++++++++++++++++++
mm/page-writeback.c | 4 +
8 files changed, 335 insertions(+), 35 deletions(-)

--
tejun