Re: [PATCH] x86/sgx: Fix deadloop in __sgx_alloc_epc_page()

[Aaron Lu <aaron.lu@intel.com>]

On Thu, Aug 29, 2024 at 08:17:53AM -0700, Dave Hansen wrote:
> Generally, I think it's a bad idea to refer to function names in
> subjects.  This, for instance would be much more informative:
> 
> 	x86/sgx: Fix deadlock in SGX NUMA node search

Indeed, will use this as subject, thanks.

> On 8/28/24 19:38, Aaron Lu wrote:
> > When current node doesn't have a EPC section configured by firmware and
> > all other EPC sections memory are used up, CPU can stuck inside the
> > while loop in __sgx_alloc_epc_page() forever and soft lockup will happen.
> > Note how nid_of_current will never equal to nid in that while loop because
> > nid_of_current is not set in sgx_numa_mask.
> > 
> > Also worth mentioning is that it's perfectly fine for firmware to not
> > seup an EPC section on a node. Setting an EPC section on each node can
> > be good for performance but that's not a requirement functionality wise.
> 
> The changelog is a little rough, but I think Kai gave some good
> suggestions.  The other thing you can do is dump the text in chatgpt (or
> whatever) and have it fix your grammar.  It actually does a pretty
> decent job.

Thanks for the suggestion.

> 
> Also, you didn't say _how_ you fixed this.  That needs to be in here.
> Something along the lines of:
> 
> 	Rework the loop to start and end on *a* node that has SGX
> 	memory.  This avoids the deadlock looking for the current SGX-
> 	lacking node to show up in the loop when it never will.

Will add this to the changelog, thanks for the write-up.

> 
> The code looks fine, so feel free to add:
> 
> Acked-by: Dave Hansen <dave.hansen@linux.intel.com>

Thanks.

> 
> Also, I do think we should probably add some kind of sanity warning to
> the SGX code in another patch.  If a node on an SGX system has CPUs and
> memory, it's very likely it will also have some EPC.  It can be
> something soft like a pr_info(), but I think it would be nice to have.

I think there are systems with valid reason to not setup an EPC section
per node, e.g. a 8 sockets system with SNC=2, there would be a total of
16 nodes and it's not possible to have one EPC section per node because
the upper limit of EPC sections is 8. I'm not sure a warning is
appropriate here, what do you think?