Re: MGLRU premature memcg OOM on slow writes

[Axel Rasmussen <axelrasmussen@google.com>]

On Mon, Mar 11, 2024 at 2:11 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Sat, Mar 9, 2024 at 3:19 AM Axel Rasmussen <axelrasmussen@google.com> wrote:
> >
> > On Thu, Feb 29, 2024 at 4:30 PM Chris Down <chris@chrisdown.name> wrote:
> > >
> > > Axel Rasmussen writes:
> > > >A couple of dumb questions. In your test, do you have any of the following
> > > >configured / enabled?
> > > >
> > > >/proc/sys/vm/laptop_mode
> > > >memory.low
> > > >memory.min
> > >
> > > None of these are enabled. The issue is trivially reproducible by writing to
> > > any slow device with memory.max enabled, but from the code it looks like MGLRU
> > > is also susceptible to this on global reclaim (although it's less likely due to
> > > page diversity).
> > >
> > > >Besides that, it looks like the place non-MGLRU reclaim wakes up the
> > > >flushers is in shrink_inactive_list() (which calls wakeup_flusher_threads()).
> > > >Since MGLRU calls shrink_folio_list() directly (from evict_folios()), I agree it
> > > >looks like it simply will not do this.
> > > >
> > > >Yosry pointed out [1], where MGLRU used to call this but stopped doing that. It
> > > >makes sense to me at least that doing writeback every time we age is too
> > > >aggressive, but doing it in evict_folios() makes some sense to me, basically to
> > > >copy the behavior the non-MGLRU path (shrink_inactive_list()) has.
> > >
> > > Thanks! We may also need reclaim_throttle(), depending on how you implement it.
> > > Current non-MGLRU behaviour on slow storage is also highly suspect in terms of
> > > (lack of) throttling after moving away from VMSCAN_THROTTLE_WRITEBACK, but one
> > > thing at a time :-)
> >
> >
> > Hmm, so I have a patch which I think will help with this situation,
> > but I'm having some trouble reproducing the problem on 6.8-rc7 (so
> > then I can verify the patch fixes it).
>
> We encountered the same premature OOM issue caused by numerous dirty pages.
> The issue disappears after we revert the commit 14aa8b2d5c2e
> "mm/mglru: don't sync disk for each aging cycle"
>
> To aid in replicating the issue, we've developed a straightforward
> script, which consistently reproduces it, even on the latest kernel.
> You can find the script provided below:
>
> ```
> #!/bin/bash
>
> MEMCG="/sys/fs/cgroup/memory/mglru"
> ENABLE=$1
>
> # Avoid waking up the flusher
> sysctl -w vm.dirty_background_bytes=$((1024 * 1024 * 1024 *4))
> sysctl -w vm.dirty_bytes=$((1024 * 1024 * 1024 *4))
>
> if [ ! -d ${MEMCG} ]; then
>         mkdir -p ${MEMCG}
> fi
>
> echo $$ > ${MEMCG}/cgroup.procs
> echo 1g > ${MEMCG}/memory.limit_in_bytes
>
> if [ $ENABLE -eq 0 ]; then
>         echo 0 > /sys/kernel/mm/lru_gen/enabled
> else
>         echo 0x7 > /sys/kernel/mm/lru_gen/enabled
> fi
>
> dd if=/dev/zero of=/data0/mglru.test bs=1M count=1023
> rm -rf /data0/mglru.test
> ```
>
> This issue disappears as well after we disable the mglru.
>
> We hope this script proves helpful in identifying and addressing the
> root cause. We eagerly await your insights and proposed fixes.

Thanks Yafang, I was able to reproduce the issue using this script.

Perhaps interestingly, I was not able to reproduce it with cgroupv2
memcgs. I know writeback semantics are quite a bit different there, so
perhaps that explains why.

Unfortunately, it also reproduces even with the commit I had in mind
(basically stealing the "if (all isolated pages are unqueued dirty) {
wakeup_flusher_threads(); reclaim_throttle(); }" from
shrink_inactive_list, and adding it to MGLRU's evict_folios()). So
I'll need to spend some more time on this; I'm planning to send
something out for testing next week.

>
> >
> > If I understand the issue right, all we should need to do is get a
> > slow filesystem, and then generate a bunch of dirty file pages on it,
> > while running in a tightly constrained memcg. To that end, I tried the
> > following script. But, in reality I seem to get little or no
> > accumulation of dirty file pages.
> >
> > I thought maybe fio does something different than rsync which you said
> > you originally tried, so I also tried rsync (copying /usr/bin into
> > this loop mount) and didn't run into an OOM situation either.
> >
> > Maybe some dirty ratio settings need tweaking or something to get the
> > behavior you see? Or maybe my test has a dumb mistake in it. :)
> >
> >
> >
> > #!/usr/bin/env bash
> >
> > echo 0 > /proc/sys/vm/laptop_mode || exit 1
> > echo y > /sys/kernel/mm/lru_gen/enabled || exit 1
> >
> > echo "Allocate disk image"
> > IMAGE_SIZE_MIB=1024
> > IMAGE_PATH=/tmp/slow.img
> > dd if=/dev/zero of=$IMAGE_PATH bs=1024k count=$IMAGE_SIZE_MIB || exit 1
> >
> > echo "Setup loop device"
> > LOOP_DEV=$(losetup --show --find $IMAGE_PATH) || exit 1
> > LOOP_BLOCKS=$(blockdev --getsize $LOOP_DEV) || exit 1
> >
> > echo "Create dm-slow"
> > DM_NAME=dm-slow
> > DM_DEV=/dev/mapper/$DM_NAME
> > echo "0 $LOOP_BLOCKS delay $LOOP_DEV 0 100" | dmsetup create $DM_NAME || exit 1
> >
> > echo "Create fs"
> > mkfs.ext4 "$DM_DEV" || exit 1
> >
> > echo "Mount fs"
> > MOUNT_PATH="/tmp/$DM_NAME"
> > mkdir -p "$MOUNT_PATH" || exit 1
> > mount -t ext4 "$DM_DEV" "$MOUNT_PATH" || exit 1
> >
> > echo "Generate dirty file pages"
> > systemd-run --wait --pipe --collect -p MemoryMax=32M \
> >         fio -name=writes -directory=$MOUNT_PATH -readwrite=randwrite \
> >         -numjobs=10 -nrfiles=90 -filesize=1048576 \
> >         -fallocate=posix \
> >         -blocksize=4k -ioengine=mmap \
> >         -direct=0 -buffered=1 -fsync=0 -fdatasync=0 -sync=0 \
> >         -runtime=300 -time_based
> >
>
>
> --
> Regards
> Yafang