Re: [PATCH] coroutine: cap per-thread local pool size

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Mar 19, 2024 at 05:54:38PM +0100, Kevin Wolf wrote:
> Am 19.03.2024 um 14:43 hat Daniel P. Berrangé geschrieben:
> > On Mon, Mar 18, 2024 at 02:34:29PM -0400, Stefan Hajnoczi wrote:
> > > The coroutine pool implementation can hit the Linux vm.max_map_count
> > > limit, causing QEMU to abort with "failed to allocate memory for stack"
> > > or "failed to set up stack guard page" during coroutine creation.
> > > 
> > > This happens because per-thread pools can grow to tens of thousands of
> > > coroutines. Each coroutine causes 2 virtual memory areas to be created.
> > 
> > This sounds quite alarming. What usage scenario is justified in
> > creating so many coroutines?
> 
> Basically we try to allow pooling coroutines for as many requests as
> there can be in flight at the same time. That is, adding a virtio-blk
> device increases the maximum pool size by num_queues * queue_size. If
> you have a guest with many CPUs, the default num_queues is relatively
> large (the bug referenced by Stefan had 64), and queue_size is 256 by
> default. That's 16k potential requests in flight per disk.

If we have more than 1 virtio-blk device, does that scale
up the max coroutines too ?

eg would 32 virtio-blks devices imply 16k * 32 -> 512k potential
requests/coroutines ?

> > IIUC, coroutine stack size is 1 MB, and so tens of thousands of
> > coroutines implies 10's of GB of memory just on stacks alone.
> 
> That's only virtual memory, though. Not sure how much of it is actually
> used in practice.

True, by default Linux wouldn't care too much about virtual memory,
Only if 'vm.overcommit_memory' is changed from its default, such
that Linux applies an overcommit ratio on RAM, then total virtual
memory would be relevant.



> > > Eventually vm.max_map_count is reached and memory-related syscalls fail.
> > 
> > On my system max_map_count is 1048576, quite alot higher than
> > 10's of 1000's. Hitting that would imply ~500,000 coroutines and
> > ~500 GB of stacks !
> 
> Did you change the configuration some time in the past, or is this just
> a newer default? I get 65530, and that's the same default number I've
> seen in the bug reports.

It turns out it is a Fedora change, rather than a kernel change:

  https://fedoraproject.org/wiki/Changes/IncreaseVmMaxMapCount

> > > diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> > > index 5fd2dbaf8b..2790959eaf 100644
> > > --- a/util/qemu-coroutine.c
> > > +++ b/util/qemu-coroutine.c
> > 
> > > +static unsigned int get_global_pool_hard_max_size(void)
> > > +{
> > > +#ifdef __linux__
> > > +    g_autofree char *contents = NULL;
> > > +    int max_map_count;
> > > +
> > > +    /*
> > > +     * Linux processes can have up to max_map_count virtual memory areas
> > > +     * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond this limit. We
> > > +     * must limit the coroutine pool to a safe size to avoid running out of
> > > +     * VMAs.
> > > +     */
> > > +    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, NULL,
> > > +                            NULL) &&
> > > +        qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
> > > +        /*
> > > +         * This is a conservative upper bound that avoids exceeding
> > > +         * max_map_count. Leave half for non-coroutine users like library
> > > +         * dependencies, vhost-user, etc. Each coroutine takes up 2 VMAs so
> > > +         * halve the amount again.
> > > +         */
> > > +        return max_map_count / 4;
> > 
> > That's 256,000 coroutines, which still sounds incredibly large
> > to me.
> 
> The whole purpose of the limitation is that you won't ever get -ENOMEM
> back, which will likely crash your VM. Even if this hard limit is high,
> that doesn't mean that it's fully used. Your setting of 1048576 probably
> means that you would never have hit the crash anyway.
> 
> Even the benchmarks that used to hit the problem don't even get close to
> this hard limit any more because the actual number of coroutines stays
> much smaller after applying this patch.

I'm more thinking about what's the worst case behaviour that a
malicious guest can inflict on QEMU, and cause unexpectedly
high memory usage in the host.

ENOMEM is bad for a friendy VM, but there's also the risk to
the host from a unfriendly VM exploiting the high limits

> 
> > > +    }
> > > +#endif
> > > +
> > > +    return UINT_MAX;
> > 
> > Why UINT_MAX as a default ?  If we can't read procfs, we should
> > assume some much smaller sane default IMHO, that corresponds to
> > what current linux default max_map_count would be.
> 
> I don't think we should artificially limit the pool size and with this
> potentially limit the performance with it even if the host could do more
> if we only allowed it to. If we can't read it from procfs, then it's
> your responsibility as a user to make sure that it's large enough for
> your VM configuration.


With regards,
Daniel
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