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

From: Qian Cai <[email-masked]>
Date: Mon, 03 Feb 2020 10:48:14 -0500

On Mon, 2020-02-03 at 16:14 +0100, Christophe Leroy wrote:
> 
> Le 02/02/2020 à 12:26, Qian Cai a écrit :
> > 
> > 
> > > On Jan 30, 2020, at 9:13 AM, Christophe Leroy <[email-masked]> wrote:
> > > 
> > > config DEBUG_VM_PGTABLE
> > >     bool "Debug arch page table for semantics compliance" if ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
> > >     depends on MMU
> > >     default 'n' if !ARCH_HAS_DEBUG_VM_PGTABLE
> > >     default 'y' if DEBUG_VM
> > 
> > Does it really necessary to potentially force all bots to run this? Syzbot, kernel test robot etc? Does it ever pay off for all their machine times there?
> > 
> 
> Machine time ?
> 
> On a 32 bits powerpc running at 132 MHz, the tests takes less than 10ms. 
> Is it worth taking the risk of not detecting faults by not selecting it 
> by default ?

The risk is quite low as Catalin mentioned this thing is not to detect
regressions but rather for arch/mm maintainers.

I do appreciate the efforts to get everyone as possible to run this thing,
so it get more notices once it is broken. However, DEBUG_VM seems like such
a generic Kconfig those days that have even been enabled by default for
Fedora Linux, so I would rather see a more sensitive default been taken
even though the test runtime is fairly quickly on a small machine for now.

> 
> [    5.656916] debug_vm_pgtable: debug_vm_pgtable: Validating 
> architecture page table helpers
> [    5.665661] debug_vm_pgtable: debug_vm_pgtable: Validated 
> architecture page table helpers