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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
