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Subject: Re: [PATCH] virtio_balloon: use non-blocking allocation

On Wed, Jan 31, 2018 at 08:13:26PM +0900, Tetsuo Handa wrote:
> Michael S. Tsirkin wrote:
> > On Tue, Jan 02, 2018 at 11:50:21PM +0900, Tetsuo Handa wrote:
> > > Commit c7cdff0e864713a0 ("virtio_balloon: fix deadlock on OOM") tried to
> > > avoid OOM lockup by moving memory allocations to outside of balloon_lock.
> > > 
> > > Now, Wei is trying to allocate far more pages outside of balloon_lock and
> > > some more memory inside of balloon_lock in order to perform efficient
> > > communication between host and guest using scatter-gather API.
> > > 
> > > Since pages allocated outside of balloon_lock are not visible to the OOM
> > > notifier path until fill_balloon() holds balloon_lock (and enqueues the
> > > pending pages), allocating more pages than now may lead to unacceptably
> > > premature OOM killer invocation.
> > > 
> > > It would be possible to make the pending pages visible to the OOM notifier
> > > path. But there is no need to try to allocate memory so hard from the
> > > beginning. As of commit 18468d93e53b037e ("mm: introduce a common
> > > interface for balloon pages mobility"), it made sense to try allocation
> > > as hard as possible. But after commit 5a10b7dbf904bfe0 ("virtio_balloon:
> > > free some memory from balloon on OOM"),
> > 
> > However, please not that this behavious is optional.
> > Can you keep the current behaviour when deflate on OOM is disabled?
> I can, for passing a flag to balloon_page_alloc() will do it.
> But do we really prefer behavior up to comment 27 of
> https://bugzilla.redhat.com/show_bug.cgi?id=1525356 ?

You show a config where so much memory is taken that guest crashes, but
hopefully in other situations it's just an application hogging memory.

So I'm sure current behaviour is not optimal for your config but the
problem with deflate on OOM is it does not restart inflating when OOM
condition goes away.  So if host *really* needs that memory it can't
enable deflate on OOM.

Crashing the kernel is of course not really useful, I wish
there was a way to avoid that while still reliably
giving as much as we can to the host.

Ideally balloon flow looks like this:
1. host needs some memory e.g. to start a new guest
2. guest gets request from host to give it back some memory
3. while request not satisfied:
	a. try to get hold of free memory
	b. free up some by flushing caches
	c. free up some by killing memory hogs

All this without crashing the guest.

What we have implemented is a rough approximation.

Deflate on OOM reduces the chance of a crash or hang but it makes the
inflate unreliable: host can no longer use the memory for another guest,
this one might request it back at any time.

What is deflate on oom good for then?  I suspect that people use deflate
on oom as poor man's page hinting.

> > 
> > 
> > > it no longer makes sense to try
> > > allocation as hard as possible, for fill_balloon() will after all have to
> > > release just allocated memory if some allocation request hits the OOM
> > > notifier path. Therefore, this patch disables __GFP_DIRECT_RECLAIM when
> > > allocating memory for inflating the balloon. Then, memory for inflating
> > > the balloon can be allocated inside balloon_lock, and we can release just
> > > allocated memory as needed.
> > > 
> > > Also, this patch adds __GFP_NOWARN, for possibility of hitting memory
> > > allocation failure is increased by removing __GFP_DIRECT_RECLAIM, which
> > > might spam the kernel log buffer. At the same time, this patch moves
> > > "puff" messages to outside of balloon_lock, for it is not a good thing to
> > > block the OOM notifier path for 1/5 of a second. (Moreover, it is better
> > > to release the workqueue and allow processing other pending items. But
> > > that change is out of this patch's scope.)
> > > 
> > > __GFP_NOMEMALLOC is currently not required because workqueue context
> > > which calls balloon_page_alloc() won't cause __gfp_pfmemalloc_flags()
> > > to return ALLOC_OOM. But since some process context might start calling
> > > balloon_page_alloc() in future, this patch does not remove
> > > 
> > > (Only compile tested. Please do runtime tests before committing.)
> > 
> > You will have to find someone to test it.
> I don't have machines with much memory.

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