Re: [PATCH v7 3/6] accel/kvm: Report the loss of a large memory page

[Peter Xu]

On Tue, Feb 11, 2025 at 10:22:38PM +0100, William Roche wrote:
> On 2/10/25 17:48, Peter Xu wrote:
> > On Fri, Feb 07, 2025 at 07:02:22PM +0100, William Roche wrote:
> > > [...]
> > > So the main reason is a KVM "weakness" with kvm_send_hwpoison_signal(), 
> > > and
> > > the second reason is to have richer error messages.
> > 
> > This seems true, and I also remember something when I looked at this
> > previously but maybe nobody tried to fix it.  ARM seems to be correct on
> > that field, otoh.
> > 
> > Is it possible we fix KVM on x86?
> 
> Yes, very probably, and it would be a kernel fix.
> This kernel modification would be needed to run on the hypervisor first to
> influence a new code in qemu able to use the SIGBUS siginfo information and
> identify the size of the page impacted (instead of using an internal
> addition to kvm API).
> But this mechanism could help to generate a large page memory error specific
> message on SIGBUS receiving.

Yes, QEMU should probably better be able to work on both old/new kernels,
even if this will be fixed.

> 
> 
> > > > 
> > > > I feel like when hwpoison becomes a serious topic, we need some more
> > > > serious reporting facility than error reports.  So that we could have 
> > > > this
> > > > as separate topic to be revisited.  It might speed up your prior patches
> > > > from not being blocked on this.
> > > 
> > > I explained why I think that error messages are important, but I don't 
> > > want
> > > to get blocked on fixing the hugepage memory recovery because of that.
> > 
> > What is the major benefit of reporting in QEMU's stderr in this case?
> 
> Such messages can be collected into VM specific log file, as any other
> error_report() message, like the existing x86 error injection messages
> reported by Qemu.
> This messages should help the administrator to better understand the
> behavior of the VM.

I'll still put "better understand the behavior of VM" into debugging
category. :)

But I agree such can be important information.  That's also why I was
curious whether it should be something like a QMP event instead.  That's a
much formal way of sending important messages.

> 
> 
> > For example, how should we consume the error reports that this patch
> > introduces?  Is it still for debugging purpose?
> 
> Its not only debugging, but it's a trace of a significant event that can
> have major consequences on the VM.
> 
> > 
> > I agree it's always better to dump something in QEMU when such happened,
> > but IIUC what I mentioned above (by monitoring QEMU ramblock setups, and
> > monitor host dmesg on any vaddr reported hwpoison) should also allow anyone
> > to deduce the page size of affected vaddr, especially if it's for debugging
> > purpose.  However I could possibly have missed the goal here..
> 
> You're right that knowing the address, the administrator can deduce what
> memory area was impacted and the associated page size. But the goal of these
> large page specific messages was to give details on the event type and
> immediately qualify the consequences.
> Using large pages can also have drawbacks, and a large page specific message
> on memory error makes that more obvious !  Not only a debug msg, but an
> indication that the VM lost an unusually large amount of its memory.
> 
> > > 
> > > If you think that not displaying a specific message for large page loss 
> > > can
> > > help to get the recovery fixed, than I can change my proposal to do so.
> > > 
> > > Early next week, I'll send a simplified version of my first 3 patches
> > > without this specific messages and without the preallocation handling in 
> > > all
> > > remap cases, so you can evaluate this possibility.
> > 
> > Yes IMHO it'll always be helpful to separate it if possible.
> 
> I'm sending now a v8 version, without the specific messages and the remap
> notification. It should fix the main recovery bug we currently have. More
> messages and a notification dealing with pre-allocation can be added in a
> second step.
> 
> Please let me know if this v8 version can be integrated without the prealloc
> and specific messages ?

IMHO fixing hugetlb page is still a progress on its own, even without any
added error message, or proactive allocation during reset.

One issue is the v8 still contains patch 3 which is for ARM kvm.. You may
need to post it separately for ARM maintainers to review & collect.  I'll
be able to queue patch 1-2.

Thanks,

-- 
Peter Xu