Re: [PATCH 0/1] prctl: allow overriding system THP policy to always

[David Hildenbrand <david@redhat.com>]

>>>> - madvise
>>>>    The sysadmin gently encourages the use of THP, but it is only
>>>> enabled when explicitly requested by the application.
> 
> And this "user mode" or "manual mode", where applications self-manage
> which parts of userspace they want to enroll.
> 
> Both madvise() and unprivileged prctl() should work here as well,
> IMO. There is no policy or security difference between them, it's just
> about granularity and usability.
> 
>>>> - never
>>>>     The sysadmin discourages the use of THP, and "its use is only permitted
>>>> with explicit approval" .
> 
> This one I don't quite agree with, and IMO conflicts with what David
> is saying as well.

Yeah ... "never" does not mean "sometimes" in my reality :)

> 
>>> "never" so far means "no thps, no exceptions". We've had serious THP
>>> issues in the past, where our workaround until we sorted out the issue
>>> for affected customers was to force-disable THPs on that system during boot.
>>
>> Right, that reflects the current behavior. What we aim to enhance is
>> by adding the requirement that "its use is only permitted with
>> explicit approval."
> 
> I think you're conflating a safety issue with a security issue.
> 
> David is saying there can be cases where the kernel is broken, and
> "never" is a production escape hatch to disable the feature until a
> kernel upgrade for the fix is possible. In such a case, it doesn't
> make sense to override this decision based on any sort of workload
> policy, privileged or not.
> 
> The way I understand you is that you want enrollment (and/or
> self-management) only for blessed applications. Because you don't
> generally trust workloads in the wild enough to switch the global
> default away from "never", given the semantics of always/madvise.

Assuming "never" means "never" and "always" means "always" ( crazy, 
right? :) ), could be make use of "madvise" mode, which essentially 
means "VM_HUGEPAGE" takes control?

We'd need

a) A way to enable THP for a process. Changing the default/vma settings 
to VM_HUGEPAGE as discussed using a prctl could work.

b) A way to ignore VM_HUGEPAGE for processes. Maybe the existing prctl 
to force-disable THPs could work?

-- 
Cheers,

David / dhildenb