linux-kernel - [PATCHSET] writeback, memcg: Implement foreign inode flushing

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Date:   Sat,  3 Aug 2019 07:01:51 -0700
From:   Tejun Heo <tj@...nel.org>
To:     axboe@...nel.dk, jack@...e.cz, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com
Cc:     cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, guro@...com, akpm@...ux-foundation.org
Subject: [PATCHSET] writeback, memcg: Implement foreign inode flushing

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