[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc(

From: David Hildenbrand
Subject: Re: [PATCH v1 3/3] util/oslib-posix: Support concurrent os_mem_prealloc() invocation
Date: Tue, 20 Jul 2021 16:27:32 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 20.07.21 16:22, Daniel P. Berrangé wrote:
On Wed, Jul 14, 2021 at 01:23:06PM +0200, David Hildenbrand wrote:
Add a mutext to protect the SIGBUS case, as we cannot mess concurrently

typo  s/mutext/mutex/

with the sigbus handler and we have to manage the global variable
sigbus_memset_context. The MADV_POPULATE_WRITE path can run

Note that page_mutex and page_cond are shared between concurrent
invocations, which shouldn't be a problem.

This is a preparation for future virtio-mem prealloc code, which will call
os_mem_prealloc() asynchronously from an iothread when handling guest

Hmm, I'm wondering how the need to temporarily play with SIGBUS
at runtime for mem preallocation will interact with the SIGBUS
handler installed by softmmu/cpus.c.

That's exactly why I came up with MADV_POPULATE_WRITE, to avoid having to mess with different kinds of sigbus at the same time. You can only get it wrong.

The SIGBUS handler the preallocation code is installed just
blindly assumes the SIGBUS is related to the preallocation
work being done. This is a fine assumption during initially
startup where we're single threaded and not running guest
CPUs. I'm less clear on whether that's a valid assumption
at runtime once guest CPUs are running.

I assume it's quite broken, for example, already when hotplugging a DIMM and prallocating memory for the memory backend.

If the sigbus_handler method in softmmu/cpus.c is doing
something important for QEMU, then why is it ok for us to
periodically disable that handler and replace it with
something else that takes a completely different action ?

I don't think it is ok. I assume it has been broken for a long time, just nobody ever ran into that race.

Of course with the madvise impl we're bypassing the SIGBUS
dance entirely. This is good for people with new kernels,
but is this SIGBUS stuff safe for older kernels ?

It remains broken with old kernels I guess. There isn't too much that we can do: disabling prealloc=on once the VM is running breaks existing use cases.

Fortunately, running into that race seems to be rare, at least I never hear reports.


David / dhildenb

reply via email to

[Prev in Thread] Current Thread [Next in Thread]