Re: [PATCH V12] mm/debug: Add tests validating architecture page table helpers

[Qian Cai <cai@xxxxxx>]

> On Jan 29, 2020, at 5:36 AM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> 
> On Tue, Jan 28, 2020 at 02:07:10PM -0500, Qian Cai wrote:
>> On Jan 28, 2020, at 12:47 PM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>>> The primary goal here is not finding regressions but having clearly
>>> defined semantics of the page table accessors across architectures. x86
>>> and arm64 are a good starting point and other architectures will be
>>> enabled as they are aligned to the same semantics.
>> 
>> This still does not answer the fundamental question. If this test is
>> simply inefficient to find bugs,
> 
> Who said this is inefficient (other than you)?

Inefficient of finding bugs. It said only found a bug or two in its lifetime?

> 
>> who wants to spend time to use it regularly? 
> 
> Arch maintainers, mm maintainers introducing new macros or assuming
> certain new semantics of the existing macros.
> 
>> If this is just one off test that may get running once in a few years
>> (when introducing a new arch), how does it justify the ongoing cost to
>> maintain it?
> 
> You are really missing the point. It's not only for a new arch but
> changes to existing arch code. And if the arch code churn in this area
> is relatively small, I'd expect a similarly small cost of maintaining
> this test.
> 
> If you only turn DEBUG_VM on once every few years, don't generalise this
> to the rest of the kernel developers (as others pointed out, this test
> is default y if DEBUG_VM).

Quite the opposite, I am running DEBUG_VM almost daily for regression
workload while I felt strongly this thing does not add any value mixing there.

So, I would suggest to decouple this away from DEBUG_VM, and clearly
document that this test is not something intended for automated regression
workloads, so those people don’t need to waste time running this.

> 
> Anyway, I think that's a pointless discussion, so not going to reply
> further (unless you have technical content to add).
> 
> -- 
> Catalin