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Tetsuo Handa

2017-Oct-16 10:58 UTC

[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()

Tetsuo Handa wrote:> Michael S. Tsirkin wrote:
> > > > 
> > > > The proper fix isn't that hard - just avoid allocations
under lock.
> > > > 
> > > > Patch posted, pls take a look.
> > > 
> > > Your patch allocates pages in order to inflate the balloon, but
> > > your patch will allow leak_balloon() to deflate the balloon.
> > > How deflating the balloon (i.e. calling leak_balloon()) makes
sense
> > > when allocating pages for inflating the balloon (i.e. calling
> > > fill_balloon()) ?
> > 
> > The idea is that fill_balloon is allocating memory with __GFP_NORETRY
> > so it will avoid disruptive actions like the OOM killer.
> > Under pressure it will normally fail and retry in half a second or so.
> > 
> > Calling leak_balloon in that situation could benefit the system as a
whole.
> > 
> > I might be misunderstanding the meaning of the relevant GFP flags,
> > pls correct me if I'm wrong.
> 
> Would you answer to below question by "yes"/"no" ?
> 
>   If leak_balloon() is called via out_of_memory(), leak_balloon()
>   will decrease "struct virtio_balloon"->num_pages.
>   But, is "struct virtio_balloon_config"->num_pages updated
when
>   leak_balloon() is called via out_of_memory() ?
> 
> Below explanation assumes that your answer is "no".
> 
> I consider that fill_balloon() is using __GFP_NORETRY is a bug.
Below are my test results using 4.14-rc5.

  http://I-love.SAKURA.ne.jp/tmp/20171016-default.log.xz
  http://I-love.SAKURA.ne.jp/tmp/20171016-deflate.log.xz

20171016-default.log.xz is without VIRTIO_BALLOON_F_DEFLATE_ON_OOM and
20171016-deflate.log.xz is with VIRTIO_BALLOON_F_DEFLATE_ON_OOM. (I used
inverting virtio_has_feature(VIRTIO_BALLOON_F_DEFLATE_ON_OOM) test
because the QEMU I'm using does not support deflate-on-oom option.)
> Consider an extreme situation that guest1 is started with 8192MB
> memory and then guest1's memory is reduced to 128MB by
> 
>   virsh qemu-monitor-command --domain guest1 --hmp 'balloon 128'
> 
> when VIRTIO_BALLOON_F_DEFLATE_ON_OOM was not negotiated.
> Of course, 128MB would be too small to operate guest1 properly.
> Since update_balloon_size_func() continues calling fill_balloon()
> until guest1's memory is reduced to 128MB, you will see flooding of
> "puff" messages (and guest1 is practically unusable because all
CPU
> resource will be wasted for unsuccessful memory reclaim attempts)
> unless the OOM killer is invoked.
20171016-default.log.xz continued printing "puff" messages until
kernel
panic caused by somebody else killing all OOM killable processes via
GFP_KERNEL allocation requests. Although fill_balloon() was printing
a lot of noises due to __GFP_NORETRY, the system made forward progress
in the form of kernel panic triggered by no more OOM killable processes.

----------
[   88.270838] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   88.503496] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   88.730058] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   88.937971] Out of memory: Kill process 669 (dhclient) score 54 or sacrifice
child
[   89.007322] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   89.234874] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   89.439735] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   89.496389] Out of memory: Kill process 853 (tuned) score 47 or sacrifice
child
[   89.663808] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   89.883812] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   90.104417] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   90.251293] Out of memory: Kill process 568 (polkitd) score 36 or sacrifice
child
[   90.326131] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   90.821722] Out of memory: Kill process 601 (NetworkManager) score 23 or
sacrifice child
[   91.293848] Out of memory: Kill process 585 (rsyslogd) score 13 or sacrifice
child
[   91.799413] Out of memory: Kill process 415 (systemd-journal) score 13 or
sacrifice child
[   91.861974] Out of memory: Kill process 987 (qmgr) score 6 or sacrifice child
[   91.925297] Out of memory: Kill process 985 (master) score 6 or sacrifice
child
[   92.254082] Out of memory: Kill process 985 (master) score 6 or sacrifice
child
[   92.464029] Out of memory: Kill process 881 (login) score 4 or sacrifice
child
[   92.467859] Out of memory: Kill process 881 (login) score 4 or sacrifice
child
[   93.490928] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   93.713220] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   93.932145] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   94.147652] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   94.363826] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   94.606404] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   94.833539] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   95.053230] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   95.267805] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   95.483789] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   98.290124] Out of memory: Kill process 595 (crond) score 3 or sacrifice
child
[   98.643588] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   98.864487] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   99.084685] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   99.299766] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   99.515745] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   99.758334] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   99.985175] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  100.204474] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  100.419757] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  100.635681] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  101.263840] Out of memory: Kill process 587 (systemd-logind) score 2 or
sacrifice child
[  101.319432] Out of memory: Kill process 586 (irqbalance) score 1 or sacrifice
child
[  101.386546] Out of memory: Kill process 569 (dbus-daemon) score 0 or
sacrifice child
[  103.805754] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  104.033073] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  104.253104] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  104.467810] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  104.683792] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  104.926409] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  105.153490] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  105.372610] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  105.587751] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  105.803719] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  108.958882] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  109.185254] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  109.404393] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  109.621584] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  109.697419] Kernel panic - not syncing: Out of memory and no killable
processes...
[  109.723710] ---[ end Kernel panic - not syncing: Out of memory and no
killable processes...
----------
> 
> What this patch is trying to handle is a situation when
> VIRTIO_BALLOON_F_DEFLATE_ON_OOM was negotiated. Once
> update_balloon_size_func() started calling fill_balloon(),
> update_balloon_size_func() will continue calling fill_balloon()
> until guest1's memory is reduced to 128MB, won't it?
> 
> Since fill_balloon() uses __GFP_IO | __GFP_FS, fill_balloon() can
> indirectly trigger out_of_memory() despite __GFP_NORETRY is specified.
> 
> When update_balloon_size_func() is running for calling fill_balloon(),
> calling leak_balloon() will increase number of pages to fill which
> fill_balloon() is supposed to fill. Leaking some pages from leak_balloon()
> via blocking_notifier_call_chain() callback could avoid invocation of the
> OOM killer for that specific moment, but it bounces back to us later
because
> number of pages to allocate later (note that update_balloon_size_func() is
> running for calling fill_balloon()) is increased by leak_balloon().
20171016-deflate.log.xz continued printing "puff" messages without any
OOM
killer messages, for fill_balloon() always inflates faster than leak_balloon()
deflates.

Since the OOM killer cannot be invoked unless leak_balloon() completely
deflates faster than fill_balloon() inflates, the guest remained unusable
(e.g. unable to login via ssh) other than printing "puff" messages.
This result was worse than 20171016-default.log.xz , for the system was
not able to make any forward progress (i.e. complete OOM lockup).

----------
[   19.866938] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   20.089350] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   20.430965] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   20.652641] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   20.894680] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   21.122063] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   21.340872] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   21.556128] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   21.772473] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   22.014960] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   24.972961] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   25.201565] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   25.420875] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   25.636081] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   25.851670] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   26.095842] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   26.322284] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   26.851691] virtio_balloon virtio3: Out of puff! Can't get 1 pages
(...snipped...)
[  211.748567] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  211.963910] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  215.157737] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  215.385239] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  215.604426] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  215.819807] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  216.036491] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  216.278718] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  216.505410] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  216.724334] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  216.940318] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  217.155560] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  220.312187] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[  220.342980] sysrq: SysRq : Resetting
----------
> 
> Thus, I don't think that avoid invoking the OOM killer by calling
leak_balloon()
> makes sense when update_balloon_size_func() is running for calling
fill_balloon().
> And this patch tries to detect it by replacing mutex_lock() with
mutex_trylock().
> 
> > Well the point of this flag is that when it's acked,
> > host knows that it's safe to inflate the balloon
> > to a large portion of guest memory and this won't
> > cause an OOM situation.
> 
> Assuming that your answer to the question is "no", I don't
think it is
> safe to inflate the balloon to a large portion of guest memory, for once
> update_balloon_size_func() started calling fill_balloon(),
> update_balloon_size_func() can not stop calling fill_balloon() even when
> blocking_notifier_call_chain() callback called leak_balloon() because
> "struct virtio_balloon_config"->num_pages will not be updated
when
> blocking_notifier_call_chain() callback called leak_balloon().
As I demonstrated above, VIRTIO_BALLOON_F_DEFLATE_ON_OOM can lead to complete
OOM lockup because out_of_memory() => fill_balloon() => out_of_memory()
=>
fill_balloon() sequence can effectively disable the OOM killer when the host
assumed that it's safe to inflate the balloon to a large portion of guest
memory and this won't cause an OOM situation.
> > I think the assumption is that it fill back up eventually
> > when guest does have some free memory.
So, my question again because such assumption is broken.
What is the expected behavior after deflating while inflating?
How should "struct virtio_balloon_config"->num_pages be
interpreted?

  struct virtio_balloon_config {
  	/* Number of pages host wants Guest to give up. */
  	__u32 num_pages;
  	/* Number of pages we've actually got in balloon. */
  	__u32 actual;
  };

If leak_balloon() from out_of_memory() should be stronger than
fill_balloon() from update_balloon_size_func(), we need to make
sure that update_balloon_size_func() stops calling fill_balloon()
when leak_balloon() was called from out_of_memory().

If fill_balloon() from update_balloon_size_func() should be
stronger than leak_balloon() from out_of_memory(), we need to make
sure that leak_balloon() from out_of_memory() is ignored when
fill_balloon() from update_balloon_size_func() is running.

Apart from vb->balloon_lock deadlock avoidance, we need to define
the expected behavior.

Michael S. Tsirkin

2017-Oct-16 17:01 UTC

head link

[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()

On Mon, Oct 16, 2017 at 07:58:29PM +0900, Tetsuo Handa
wrote:> Tetsuo Handa wrote:
> > Michael S. Tsirkin wrote:
> > > > > 
> > > > > The proper fix isn't that hard - just avoid
allocations under lock.
> > > > > 
> > > > > Patch posted, pls take a look.
> > > > 
> > > > Your patch allocates pages in order to inflate the balloon,
but
> > > > your patch will allow leak_balloon() to deflate the balloon.
> > > > How deflating the balloon (i.e. calling leak_balloon())
makes sense
> > > > when allocating pages for inflating the balloon (i.e.
calling
> > > > fill_balloon()) ?
> > > 
> > > The idea is that fill_balloon is allocating memory with
__GFP_NORETRY
> > > so it will avoid disruptive actions like the OOM killer.
> > > Under pressure it will normally fail and retry in half a second
or so.
> > > 
> > > Calling leak_balloon in that situation could benefit the system
as a whole.
> > > 
> > > I might be misunderstanding the meaning of the relevant GFP
flags,
> > > pls correct me if I'm wrong.
> > 
> > Would you answer to below question by "yes"/"no" ?
> > 
> >   If leak_balloon() is called via out_of_memory(), leak_balloon()
> >   will decrease "struct virtio_balloon"->num_pages.
> >   But, is "struct virtio_balloon_config"->num_pages
updated when
> >   leak_balloon() is called via out_of_memory() ?
> > 
> > Below explanation assumes that your answer is "no".
> > 
> > I consider that fill_balloon() is using __GFP_NORETRY is a bug.
> 
> Below are my test results using 4.14-rc5.
> 
>   http://I-love.SAKURA.ne.jp/tmp/20171016-default.log.xz
>   http://I-love.SAKURA.ne.jp/tmp/20171016-deflate.log.xz
> 
> 20171016-default.log.xz is without VIRTIO_BALLOON_F_DEFLATE_ON_OOM and
> 20171016-deflate.log.xz is with VIRTIO_BALLOON_F_DEFLATE_ON_OOM. (I used
> inverting virtio_has_feature(VIRTIO_BALLOON_F_DEFLATE_ON_OOM) test
> because the QEMU I'm using does not support deflate-on-oom option.)
> 
> > Consider an extreme situation that guest1 is started with 8192MB
> > memory and then guest1's memory is reduced to 128MB by
> > 
> >   virsh qemu-monitor-command --domain guest1 --hmp 'balloon
128'
> > 
> > when VIRTIO_BALLOON_F_DEFLATE_ON_OOM was not negotiated.
> > Of course, 128MB would be too small to operate guest1 properly.
> > Since update_balloon_size_func() continues calling fill_balloon()
> > until guest1's memory is reduced to 128MB, you will see flooding
of
> > "puff" messages (and guest1 is practically unusable because
all CPU
> > resource will be wasted for unsuccessful memory reclaim attempts)
> > unless the OOM killer is invoked.
> 
> 20171016-default.log.xz continued printing "puff" messages until
kernel
> panic caused by somebody else killing all OOM killable processes via
> GFP_KERNEL allocation requests. Although fill_balloon() was printing
> a lot of noises due to __GFP_NORETRY, the system made forward progress
> in the form of kernel panic triggered by no more OOM killable processes.
> 
> ----------
> [   88.270838] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   88.503496] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   88.730058] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   88.937971] Out of memory: Kill process 669 (dhclient) score 54 or
sacrifice child
> [   89.007322] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   89.234874] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   89.439735] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   89.496389] Out of memory: Kill process 853 (tuned) score 47 or
sacrifice child
> [   89.663808] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   89.883812] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   90.104417] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   90.251293] Out of memory: Kill process 568 (polkitd) score 36 or
sacrifice child
> [   90.326131] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   90.821722] Out of memory: Kill process 601 (NetworkManager) score 23 or
sacrifice child
> [   91.293848] Out of memory: Kill process 585 (rsyslogd) score 13 or
sacrifice child
> [   91.799413] Out of memory: Kill process 415 (systemd-journal) score 13
or sacrifice child
> [   91.861974] Out of memory: Kill process 987 (qmgr) score 6 or sacrifice
child
> [   91.925297] Out of memory: Kill process 985 (master) score 6 or
sacrifice child
> [   92.254082] Out of memory: Kill process 985 (master) score 6 or
sacrifice child
> [   92.464029] Out of memory: Kill process 881 (login) score 4 or sacrifice
child
> [   92.467859] Out of memory: Kill process 881 (login) score 4 or sacrifice
child
> [   93.490928] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   93.713220] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   93.932145] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   94.147652] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   94.363826] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   94.606404] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   94.833539] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   95.053230] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   95.267805] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   95.483789] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   98.290124] Out of memory: Kill process 595 (crond) score 3 or sacrifice
child
> [   98.643588] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   98.864487] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   99.084685] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   99.299766] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   99.515745] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   99.758334] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   99.985175] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  100.204474] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  100.419757] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  100.635681] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  101.263840] Out of memory: Kill process 587 (systemd-logind) score 2 or
sacrifice child
> [  101.319432] Out of memory: Kill process 586 (irqbalance) score 1 or
sacrifice child
> [  101.386546] Out of memory: Kill process 569 (dbus-daemon) score 0 or
sacrifice child
> [  103.805754] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  104.033073] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  104.253104] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  104.467810] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  104.683792] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  104.926409] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  105.153490] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  105.372610] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  105.587751] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  105.803719] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  108.958882] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  109.185254] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  109.404393] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  109.621584] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  109.697419] Kernel panic - not syncing: Out of memory and no killable
processes...
> [  109.723710] ---[ end Kernel panic - not syncing: Out of memory and no
killable processes...
> ----------
> 
> > 
> > What this patch is trying to handle is a situation when
> > VIRTIO_BALLOON_F_DEFLATE_ON_OOM was negotiated. Once
> > update_balloon_size_func() started calling fill_balloon(),
> > update_balloon_size_func() will continue calling fill_balloon()
> > until guest1's memory is reduced to 128MB, won't it?
> > 
> > Since fill_balloon() uses __GFP_IO | __GFP_FS, fill_balloon() can
> > indirectly trigger out_of_memory() despite __GFP_NORETRY is specified.
> > 
> > When update_balloon_size_func() is running for calling fill_balloon(),
> > calling leak_balloon() will increase number of pages to fill which
> > fill_balloon() is supposed to fill. Leaking some pages from
leak_balloon()
> > via blocking_notifier_call_chain() callback could avoid invocation of
the
> > OOM killer for that specific moment, but it bounces back to us later
because
> > number of pages to allocate later (note that
update_balloon_size_func() is
> > running for calling fill_balloon()) is increased by leak_balloon().
> 
> 20171016-deflate.log.xz continued printing "puff" messages
without any OOM
> killer messages, for fill_balloon() always inflates faster than
leak_balloon()
> deflates.
> 
> Since the OOM killer cannot be invoked unless leak_balloon() completely
> deflates faster than fill_balloon() inflates, the guest remained unusable
> (e.g. unable to login via ssh) other than printing "puff"
messages.
> This result was worse than 20171016-default.log.xz , for the system was
> not able to make any forward progress (i.e. complete OOM lockup).
> 
> ----------
> [   19.866938] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   20.089350] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   20.430965] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   20.652641] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   20.894680] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   21.122063] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   21.340872] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   21.556128] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   21.772473] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   22.014960] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   24.972961] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   25.201565] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   25.420875] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   25.636081] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   25.851670] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   26.095842] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   26.322284] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [   26.851691] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> (...snipped...)
> [  211.748567] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  211.963910] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  215.157737] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  215.385239] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  215.604426] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  215.819807] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  216.036491] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  216.278718] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  216.505410] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  216.724334] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  216.940318] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  217.155560] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  220.312187] virtio_balloon virtio3: Out of puff! Can't get 1 pages
> [  220.342980] sysrq: SysRq : Resetting
> ----------
> 
> > 
> > Thus, I don't think that avoid invoking the OOM killer by calling
leak_balloon()
> > makes sense when update_balloon_size_func() is running for calling
fill_balloon().
> > And this patch tries to detect it by replacing mutex_lock() with
mutex_trylock().
> > 
> > > Well the point of this flag is that when it's acked,
> > > host knows that it's safe to inflate the balloon
> > > to a large portion of guest memory and this won't
> > > cause an OOM situation.
> > 
> > Assuming that your answer to the question is "no", I
don't think it is
> > safe to inflate the balloon to a large portion of guest memory, for
once
> > update_balloon_size_func() started calling fill_balloon(),
> > update_balloon_size_func() can not stop calling fill_balloon() even
when
> > blocking_notifier_call_chain() callback called leak_balloon() because
> > "struct virtio_balloon_config"->num_pages will not be
updated when
> > blocking_notifier_call_chain() callback called leak_balloon().
> 
> As I demonstrated above, VIRTIO_BALLOON_F_DEFLATE_ON_OOM can lead to
complete
> OOM lockup because out_of_memory() => fill_balloon() =>
out_of_memory() =>
> fill_balloon() sequence can effectively disable the OOM killer when the
host
> assumed that it's safe to inflate the balloon to a large portion of
guest
> memory and this won't cause an OOM situation.
I agree. But I wonder how did the contributors use it in their systems.

> > > I think the assumption is that it fill back up eventually
> > > when guest does have some free memory.
> 
> So, my question again because such assumption is broken.
> What is the expected behavior after deflating while inflating?
> How should "struct virtio_balloon_config"->num_pages be
interpreted?
> 
>   struct virtio_balloon_config {
>   	/* Number of pages host wants Guest to give up. */
>   	__u32 num_pages;
>   	/* Number of pages we've actually got in balloon. */
>   	__u32 actual;
>   };
> 
> If leak_balloon() from out_of_memory() should be stronger than
> fill_balloon() from update_balloon_size_func(), we need to make
> sure that update_balloon_size_func() stops calling fill_balloon()
> when leak_balloon() was called from out_of_memory().
I think that's the case. Question is, when can we inflate again?
> If fill_balloon() from update_balloon_size_func() should be
> stronger than leak_balloon() from out_of_memory(), we need to make
> sure that leak_balloon() from out_of_memory() is ignored when
> fill_balloon() from update_balloon_size_func() is running.
> 
> Apart from vb->balloon_lock deadlock avoidance, we need to define
> the expected behavior.

-- 
MST

Tetsuo Handa

2017-Oct-18 10:59 UTC

head link

[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()

Tetsuo Handa wrote:> 20171016-deflate.log.xz continued printing "puff" messages
without any OOM
> killer messages, for fill_balloon() always inflates faster than
leak_balloon()
> deflates.
> 
> Since the OOM killer cannot be invoked unless leak_balloon() completely
> deflates faster than fill_balloon() inflates, the guest remained unusable
> (e.g. unable to login via ssh) other than printing "puff"
messages.
> This result was worse than 20171016-default.log.xz , for the system was
> not able to make any forward progress (i.e. complete OOM lockup).
I tested further and found that it is not complete OOM lockup.

It turned out that the reason of being unable to login via ssh was that fork()
was failing because __vm_enough_memory() was failing because
/proc/sys/vm/overcommit_memory was set to 0. Although virtio_balloon driver
was ready to release pages if asked via virtballoon_oom_notify() from
out_of_memory(), __vm_enough_memory() was not able to take such pages into
account. As a result, operations which need to use fork() were failing without
calling out_of_memory().
( http://lkml.kernel.org/r/201710181954.FHH51594.MtFOFLOQFSOHVJ at
I-love.SAKURA.ne.jp )

Do you see anything wrong with the patch I used for emulating
VIRTIO_BALLOON_F_DEFLATE_ON_OOM path (shown below) ?

----------------------------------------
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index f0b3a0b..a679ac2 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -164,7 +164,7 @@ static unsigned fill_balloon(struct virtio_balloon *vb,
size_t num)
 		}
 		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
 		vb->num_pages += VIRTIO_BALLOON_PAGES_PER_PAGE;
-		if (!virtio_has_feature(vb->vdev,
+		if (virtio_has_feature(vb->vdev,
 					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 			adjust_managed_page_count(page, -1);
 	}
@@ -184,7 +184,7 @@ static void release_pages_balloon(struct virtio_balloon *vb,
 	struct page *page, *next;
 
 	list_for_each_entry_safe(page, next, pages, lru) {
-		if (!virtio_has_feature(vb->vdev,
+		if (virtio_has_feature(vb->vdev,
 					VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 			adjust_managed_page_count(page, 1);
 		list_del(&page->lru);
@@ -363,7 +363,7 @@ static int virtballoon_oom_notify(struct notifier_block
*self,
 	unsigned num_freed_pages;
 
 	vb = container_of(self, struct virtio_balloon, nb);
-	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
+	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 		return NOTIFY_OK;
 
 	freed = parm;
----------------------------------------
> As I demonstrated above, VIRTIO_BALLOON_F_DEFLATE_ON_OOM can lead to
complete
> OOM lockup because out_of_memory() => fill_balloon() =>
out_of_memory() =>
> fill_balloon() sequence can effectively disable the OOM killer when the
host
> assumed that it's safe to inflate the balloon to a large portion of
guest
> memory and this won't cause an OOM situation.
The other problem is that, although it is not complete OOM lockup, it is too
slow to wait if we hit out_of_memory() => fill_balloon() =>
out_of_memory() =>
fill_balloon() sequence.
> If leak_balloon() from out_of_memory() should be stronger than
> fill_balloon() from update_balloon_size_func(), we need to make
> sure that update_balloon_size_func() stops calling fill_balloon()
> when leak_balloon() was called from out_of_memory().
I tried below patch to reduce the possibility of hitting out_of_memory() =>
fill_balloon() => out_of_memory() => fill_balloon() sequence.

----------------------------------------
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index a679ac2..9037fee 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -57,7 +57,7 @@ struct virtio_balloon {
 
 	/* The balloon servicing is delegated to a freezable workqueue. */
 	struct work_struct update_balloon_stats_work;
-	struct work_struct update_balloon_size_work;
+	struct delayed_work update_balloon_size_work;
 
 	/* Prevent updating balloon when it is being canceled. */
 	spinlock_t stop_update_lock;
@@ -88,6 +88,7 @@ struct virtio_balloon {
 
 	/* To register callback in oom notifier call chain */
 	struct notifier_block nb;
+	struct timer_list deflate_on_oom_timer;
 };
 
 static struct virtio_device_id id_table[] = {
@@ -141,7 +142,8 @@ static void set_page_pfns(struct virtio_balloon *vb,
 					  page_to_balloon_pfn(page) + i);
 }
 
-static unsigned fill_balloon(struct virtio_balloon *vb, size_t num)
+static unsigned fill_balloon(struct virtio_balloon *vb, size_t num,
+			     unsigned long *delay)
 {
 	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
 	unsigned num_allocated_pages;
@@ -152,14 +154,21 @@ static unsigned fill_balloon(struct virtio_balloon *vb,
size_t num)
 	mutex_lock(&vb->balloon_lock);
 	for (vb->num_pfns = 0; vb->num_pfns < num;
 	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
-		struct page *page = balloon_page_enqueue(vb_dev_info);
+		struct page *page;
+
+		if (timer_pending(&vb->deflate_on_oom_timer)) {
+			/* Wait for hold off timer expiracy. */
+			*delay = HZ;
+			break;
+		}
+		page = balloon_page_enqueue(vb_dev_info);
 
 		if (!page) {
 			dev_info_ratelimited(&vb->vdev->dev,
 					     "Out of puff! Can't get %u pages\n",
 					     VIRTIO_BALLOON_PAGES_PER_PAGE);
 			/* Sleep for at least 1/5 of a second before retry. */
-			msleep(200);
+			*delay = HZ / 5;
 			break;
 		}
 		set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
@@ -310,7 +319,8 @@ static void virtballoon_changed(struct virtio_device *vdev)
 
 	spin_lock_irqsave(&vb->stop_update_lock, flags);
 	if (!vb->stop_update)
-		queue_work(system_freezable_wq, &vb->update_balloon_size_work);
+		queue_delayed_work(system_freezable_wq,
+				   &vb->update_balloon_size_work, 0);
 	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
 }
 
@@ -366,9 +376,13 @@ static int virtballoon_oom_notify(struct notifier_block
*self,
 	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_DEFLATE_ON_OOM))
 		return NOTIFY_OK;
 
+	/* Hold off fill_balloon() for 60 seconds. */
+	mod_timer(&vb->deflate_on_oom_timer, jiffies + 60 * HZ);
 	freed = parm;
 	num_freed_pages = leak_balloon(vb, oom_pages);
 	update_balloon_size(vb);
+	dev_info_ratelimited(&vb->vdev->dev, "Released %u pages.
Remains %u pages.\n",
+			     num_freed_pages, vb->num_pages);
 	*freed += num_freed_pages;
 
 	return NOTIFY_OK;
@@ -387,19 +401,21 @@ static void update_balloon_size_func(struct work_struct
*work)
 {
 	struct virtio_balloon *vb;
 	s64 diff;
+	unsigned long delay = 0;
 
-	vb = container_of(work, struct virtio_balloon,
+	vb = container_of(to_delayed_work(work), struct virtio_balloon,
 			  update_balloon_size_work);
 	diff = towards_target(vb);
 
 	if (diff > 0)
-		diff -= fill_balloon(vb, diff);
+		diff -= fill_balloon(vb, diff, &delay);
 	else if (diff < 0)
 		diff += leak_balloon(vb, -diff);
 	update_balloon_size(vb);
 
 	if (diff)
-		queue_work(system_freezable_wq, work);
+		queue_delayed_work(system_freezable_wq, to_delayed_work(work),
+				   delay);
 }
 
 static int init_vqs(struct virtio_balloon *vb)
@@ -521,6 +537,10 @@ static struct dentry *balloon_mount(struct file_system_type
*fs_type,
 
 #endif /* CONFIG_BALLOON_COMPACTION */
 
+static void timer_expired(unsigned long unused)
+{
+}
+
 static int virtballoon_probe(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb;
@@ -539,7 +559,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	}
 
 	INIT_WORK(&vb->update_balloon_stats_work, update_balloon_stats_func);
-	INIT_WORK(&vb->update_balloon_size_work, update_balloon_size_func);
+	INIT_DELAYED_WORK(&vb->update_balloon_size_work,
+			  update_balloon_size_func);
 	spin_lock_init(&vb->stop_update_lock);
 	vb->stop_update = false;
 	vb->num_pages = 0;
@@ -553,6 +574,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	if (err)
 		goto out_free_vb;
 
+	setup_timer(&vb->deflate_on_oom_timer, timer_expired, 0);
 	vb->nb.notifier_call = virtballoon_oom_notify;
 	vb->nb.priority = VIRTBALLOON_OOM_NOTIFY_PRIORITY;
 	err = register_oom_notifier(&vb->nb);
@@ -564,6 +586,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	if (IS_ERR(balloon_mnt)) {
 		err = PTR_ERR(balloon_mnt);
 		unregister_oom_notifier(&vb->nb);
+		del_timer_sync(&vb->deflate_on_oom_timer);
 		goto out_del_vqs;
 	}
 
@@ -573,6 +596,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		err = PTR_ERR(vb->vb_dev_info.inode);
 		kern_unmount(balloon_mnt);
 		unregister_oom_notifier(&vb->nb);
+		del_timer_sync(&vb->deflate_on_oom_timer);
 		vb->vb_dev_info.inode = NULL;
 		goto out_del_vqs;
 	}
@@ -611,11 +635,12 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	struct virtio_balloon *vb = vdev->priv;
 
 	unregister_oom_notifier(&vb->nb);
+	del_timer_sync(&vb->deflate_on_oom_timer);
 
 	spin_lock_irq(&vb->stop_update_lock);
 	vb->stop_update = true;
 	spin_unlock_irq(&vb->stop_update_lock);
-	cancel_work_sync(&vb->update_balloon_size_work);
+	cancel_delayed_work_sync(&vb->update_balloon_size_work);
 	cancel_work_sync(&vb->update_balloon_stats_work);
 
 	remove_common(vb);
----------------------------------------

While response was better than now, inflating again spoiled the effort.
Retrying to inflate until allocation fails is already too painful.

Complete log is at http://I-love.SAKURA.ne.jp/tmp/20171018-deflate.log.xz .
----------------------------------------
[   19.529096] kworker/0:2: page allocation failure: order:0,
mode:0x14310ca(GFP_HIGHUSER_MOVABLE|__GFP_NORETRY|__GFP_NOMEMALLOC),
nodemask=(null)
[   19.530721] kworker/0:2 cpuset=/ mems_allowed=0
[   19.531581] CPU: 0 PID: 111 Comm: kworker/0:2 Not tainted 4.14.0-rc5+ #302
[   19.532397] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[   19.533285] Workqueue: events_freezable update_balloon_size_func
[virtio_balloon]
[   19.534143] Call Trace:
[   19.535015]  dump_stack+0x63/0x87
[   19.535844]  warn_alloc+0x114/0x1c0
[   19.536667]  __alloc_pages_slowpath+0x9a6/0xba7
[   19.537491]  ? sched_clock_cpu+0x11/0xb0
[   19.538311]  __alloc_pages_nodemask+0x26a/0x290
[   19.539188]  alloc_pages_current+0x6a/0xb0
[   19.540004]  balloon_page_enqueue+0x25/0xf0
[   19.540818]  update_balloon_size_func+0xe1/0x260 [virtio_balloon]
[   19.541626]  process_one_work+0x149/0x360
[   19.542417]  worker_thread+0x4d/0x3c0
[   19.543186]  kthread+0x109/0x140
[   19.543930]  ? rescuer_thread+0x380/0x380
[   19.544716]  ? kthread_park+0x60/0x60
[   19.545426]  ret_from_fork+0x25/0x30
[   19.546141] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   19.547903] virtio_balloon virtio3: Released 256 pages. Remains 1984834
pages.
[   19.659660] virtio_balloon virtio3: Released 256 pages. Remains 1984578
pages.
[   21.891392] virtio_balloon virtio3: Released 256 pages. Remains 1984322
pages.
[   21.894719] virtio_balloon virtio3: Released 256 pages. Remains 1984066
pages.
[   22.490131] virtio_balloon virtio3: Released 256 pages. Remains 1983810
pages.
[   31.939666] virtio_balloon virtio3: Released 256 pages. Remains 1983554
pages.
[   95.524753] kworker/0:2: page allocation failure: order:0,
mode:0x14310ca(GFP_HIGHUSER_MOVABLE|__GFP_NORETRY|__GFP_NOMEMALLOC),
nodemask=(null)
[   95.525641] kworker/0:2 cpuset=/ mems_allowed=0
[   95.526110] CPU: 0 PID: 111 Comm: kworker/0:2 Not tainted 4.14.0-rc5+ #302
[   95.526552] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[   95.527018] Workqueue: events_freezable update_balloon_size_func
[virtio_balloon]
[   95.527492] Call Trace:
[   95.527969]  dump_stack+0x63/0x87
[   95.528469]  warn_alloc+0x114/0x1c0
[   95.528922]  __alloc_pages_slowpath+0x9a6/0xba7
[   95.529388]  ? qxl_image_free_objects+0x56/0x60 [qxl]
[   95.529849]  ? qxl_draw_opaque_fb+0x102/0x3a0 [qxl]
[   95.530315]  __alloc_pages_nodemask+0x26a/0x290
[   95.530777]  alloc_pages_current+0x6a/0xb0
[   95.531243]  balloon_page_enqueue+0x25/0xf0
[   95.531703]  update_balloon_size_func+0xe1/0x260 [virtio_balloon]
[   95.532180]  process_one_work+0x149/0x360
[   95.532645]  worker_thread+0x4d/0x3c0
[   95.533143]  kthread+0x109/0x140
[   95.533622]  ? rescuer_thread+0x380/0x380
[   95.534100]  ? kthread_park+0x60/0x60
[   95.534568]  ret_from_fork+0x25/0x30
[   95.535093] warn_alloc_show_mem: 1 callbacks suppressed
[   95.535093] Mem-Info:
[   95.536072] active_anon:11171 inactive_anon:2084 isolated_anon:0
[   95.536072]  active_file:8 inactive_file:70 isolated_file:0
[   95.536072]  unevictable:0 dirty:0 writeback:0 unstable:0
[   95.536072]  slab_reclaimable:3554 slab_unreclaimable:6848
[   95.536072]  mapped:588 shmem:2144 pagetables:749 bounce:0
[   95.536072]  free:25859 free_pcp:72 free_cma:0
[   95.538922] Node 0 active_anon:44684kB inactive_anon:8336kB active_file:32kB
inactive_file:280kB unevictable:0kB isolated(anon):0kB isolated(file):0kB
mapped:2352kB dirty:0kB writeback:0kB shmem:8576kB shmem_thp: 0kB
shmem_pmdmapped: 0kB anon_thp: 10240kB writeback_tmp:0kB unstable:0kB
all_unreclaimable? no
[   95.540516] Node 0 DMA free:15900kB min:132kB low:164kB high:196kB
active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB
unevictable:0kB writepending:0kB present:15992kB managed:15908kB mlocked:0kB
kernel_stack:0kB pagetables:0kB bounce:0kB free_pcp:0kB local_pcp:0kB
free_cma:0kB
[   95.542325] lowmem_reserve[]: 0 2954 7925 7925 7925
[   95.543020] Node 0 DMA32 free:44748kB min:25144kB low:31428kB high:37712kB
active_anon:0kB inactive_anon:0kB active_file:0kB inactive_file:0kB
unevictable:0kB writepending:0kB present:3129308kB managed:3063740kB mlocked:0kB
kernel_stack:0kB pagetables:0kB bounce:0kB free_pcp:0kB local_pcp:0kB
free_cma:0kB
[   95.544949] lowmem_reserve[]: 0 0 4970 4970 4970
[   95.545624] Node 0 Normal free:42788kB min:42304kB low:52880kB high:63456kB
active_anon:44684kB inactive_anon:8336kB active_file:32kB inactive_file:108kB
unevictable:0kB writepending:0kB present:5242880kB managed:5093540kB mlocked:0kB
kernel_stack:1984kB pagetables:2996kB bounce:0kB free_pcp:372kB local_pcp:220kB
free_cma:0kB
[   95.547739] lowmem_reserve[]: 0 0 0 0 0
[   95.548464] Node 0 DMA: 1*4kB (U) 1*8kB (U) 1*16kB (U) 0*32kB 2*64kB (U)
1*128kB (U) 1*256kB (U) 0*512kB 1*1024kB (U) 1*2048kB (M) 3*4096kB (M) = 15900kB
[   95.549988] Node 0 DMA32: 3*4kB (UM) 4*8kB (UM) 2*16kB (U) 2*32kB (U) 1*64kB
(U) 0*128kB 2*256kB (UM) 2*512kB (UM) 2*1024kB (UM) 2*2048kB (UM) 9*4096kB (M) =
44748kB
[   95.551551] Node 0 Normal: 925*4kB (UME) 455*8kB (UME) 349*16kB (UME)
137*32kB (UME) 37*64kB (UME) 23*128kB (UME) 8*256kB (UME) 5*512kB (UM) 3*1024kB
(UM) 6*2048kB (U) 0*4096kB = 42588kB
[   95.553147] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0
hugepages_size=2048kB
[   95.554030] 2224 total pagecache pages
[   95.554874] 0 pages in swap cache
[   95.555667] Swap cache stats: add 0, delete 0, find 0/0
[   95.556469] Free swap  = 0kB
[   95.557280] Total swap = 0kB
[   95.558079] 2097045 pages RAM
[   95.558856] 0 pages HighMem/MovableOnly
[   95.559652] 53748 pages reserved
[   95.560444] 0 pages cma reserved
[   95.561262] 0 pages hwpoisoned
[   95.562086] virtio_balloon virtio3: Out of puff! Can't get 1 pages
[   95.565779] virtio_balloon virtio3: Released 256 pages. Remains 1984947
pages.
[   96.265255] virtio_balloon virtio3: Released 256 pages. Remains 1984691
pages.
[  105.498910] virtio_balloon virtio3: Released 256 pages. Remains 1984435
pages.
[  105.500518] virtio_balloon virtio3: Released 256 pages. Remains 1984179
pages.
[  105.520034] virtio_balloon virtio3: Released 256 pages. Remains 1983923
pages.
----------------------------------------

Michael S. Tsirkin wrote:> I think that's the case. Question is, when can we inflate again?
I think that it is when the host explicitly asked again, for
VIRTIO_BALLOON_F_DEFLATE_ON_OOM path does not schedule for later inflation.

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