Re: [PATCH V12 0/3] Charge loop device i/o to issuing cgroup

[Johannes Weiner]

It looks like all feedback has been addressed and there hasn't been
any new activity on it in a while.

As per the suggestion last time [1], Andrew, Jens, could this go
through the -mm tree to deal with the memcg conflicts?

[1] https://lore.kernel.org/lkml/CALvZod6FMQQC17Zsu9xoKs=dFWaJdMC2Qk3YiDPUUQHx8teLYg@xxxxxxxxxxxxxx/

On Fri, Apr 02, 2021 at 12:16:31PM -0700, Dan Schatzberg wrote:
> No major changes, rebased on top of latest mm tree
> 
> Changes since V12:
> 
> * Small change to get_mem_cgroup_from_mm to avoid needing
>   get_active_memcg
> 
> Changes since V11:
> 
> * Removed WQ_MEM_RECLAIM flag from loop workqueue. Technically, this
>   can be driven by writeback, but this was causing a warning in xfs
>   and likely other filesystems aren't equipped to be driven by reclaim
>   at the VFS layer.
> * Included a small fix from Colin Ian King.
> * reworked get_mem_cgroup_from_mm to institute the necessary charge
>   priority.
> 
> Changes since V10:
> 
> * Added page-cache charging to mm: Charge active memcg when no mm is set
> 
> Changes since V9:
> 
> * Rebased against linus's branch which now includes Roman Gushchin's
>   patch this series is based off of
> 
> Changes since V8:
> 
> * Rebased on top of Roman Gushchin's patch
>   (https://lkml.org/lkml/2020/8/21/1464) which provides the nesting
>   support for setting active memcg. Dropped the patch from this series
>   that did the same thing.
> 
> Changes since V7:
> 
> * Rebased against linus's branch
> 
> Changes since V6:
> 
> * Added separate spinlock for worker synchronization
> * Minor style changes
> 
> Changes since V5:
> 
> * Fixed a missing css_put when failing to allocate a worker
> * Minor style changes
> 
> Changes since V4:
> 
> Only patches 1 and 2 have changed.
> 
> * Fixed irq lock ordering bug
> * Simplified loop detach
> * Added support for nesting memalloc_use_memcg
> 
> Changes since V3:
> 
> * Fix race on loop device destruction and deferred worker cleanup
> * Ensure charge on shmem_swapin_page works just like getpage
> * Minor style changes
> 
> Changes since V2:
> 
> * Deferred destruction of workqueue items so in the common case there
>   is no allocation needed
> 
> Changes since V1:
> 
> * Split out and reordered patches so cgroup charging changes are
>   separate from kworker -> workqueue change
> 
> * Add mem_css to struct loop_cmd to simplify logic
> 
> The loop device runs all i/o to the backing file on a separate kworker
> thread which results in all i/o being charged to the root cgroup. This
> allows a loop device to be used to trivially bypass resource limits
> and other policy. This patch series fixes this gap in accounting.
> 
> A simple script to demonstrate this behavior on cgroupv2 machine:
> 
> '''
> #!/bin/bash
> set -e
> 
> CGROUP=/sys/fs/cgroup/test.slice
> LOOP_DEV=/dev/loop0
> 
> if [[ ! -d $CGROUP ]]
> then
>     sudo mkdir $CGROUP
> fi
> 
> grep oom_kill $CGROUP/memory.events
> 
> # Set a memory limit, write more than that limit to tmpfs -> OOM kill
> sudo unshare -m bash -c "
> echo \$\$ > $CGROUP/cgroup.procs;
> echo 0 > $CGROUP/memory.swap.max;
> echo 64M > $CGROUP/memory.max;
> mount -t tmpfs -o size=512m tmpfs /tmp;
> dd if=/dev/zero of=/tmp/file bs=1M count=256" || true
> 
> grep oom_kill $CGROUP/memory.events
> 
> # Set a memory limit, write more than that limit through loopback
> # device -> no OOM kill
> sudo unshare -m bash -c "
> echo \$\$ > $CGROUP/cgroup.procs;
> echo 0 > $CGROUP/memory.swap.max;
> echo 64M > $CGROUP/memory.max;
> mount -t tmpfs -o size=512m tmpfs /tmp;
> truncate -s 512m /tmp/backing_file
> losetup $LOOP_DEV /tmp/backing_file
> dd if=/dev/zero of=$LOOP_DEV bs=1M count=256;
> losetup -D $LOOP_DEV" || true
> 
> grep oom_kill $CGROUP/memory.events
> '''
> 
> Naively charging cgroups could result in priority inversions through
> the single kworker thread in the case where multiple cgroups are
> reading/writing to the same loop device. This patch series does some
> minor modification to the loop driver so that each cgroup can make
> forward progress independently to avoid this inversion.
> 
> With this patch series applied, the above script triggers OOM kills
> when writing through the loop device as expected.
> 
> Dan Schatzberg (3):
>   loop: Use worker per cgroup instead of kworker
>   mm: Charge active memcg when no mm is set
>   loop: Charge i/o to mem and blk cg
> 
>  drivers/block/loop.c       | 244 ++++++++++++++++++++++++++++++-------
>  drivers/block/loop.h       |  15 ++-
>  include/linux/memcontrol.h |   6 +
>  kernel/cgroup/cgroup.c     |   1 +
>  mm/filemap.c               |   2 +-
>  mm/memcontrol.c            |  49 +++++---
>  mm/shmem.c                 |   4 +-
>  7 files changed, 253 insertions(+), 68 deletions(-)
> 
> -- 
> 2.30.2
> 
>