Re: [LKP] Re: [mm/debug] fa6726c1e7: kernel_BUG_at_include/linux/mm.h

[Christophe Leroy]

Le 29/04/2020 Ã 20:15, Catalin Marinas a ÃcritÂ:
> On Wed, Apr 29, 2020 at 08:52:25PM +0800, Chen, Rong A wrote:
> > On 4/29/2020 11:28 AM, Anshuman Khandual wrote:
> > > On 04/28/2020 02:51 PM, Catalin Marinas wrote:
> > > > On Tue, Apr 28, 2020 at 04:41:11AM -0400, Qian Cai wrote:
> > > > > On Apr 28, 2020, at 1:54 AM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote:
> > > > > > That is true. There is a slight change in the rules, making it explicit yes
> > > > > > only when both ARCH_HAS_DEBUG_VM_PGTABLE and DEBUG_VM are enabled.
> > > > > >
> > > > > > +config DEBUG_VM_PGTABLE
> > > > > > +    bool "Debug arch page table for semantics compliance"
> > > > > > +    depends on MMU
> > > > > > +    depends on !IA64 && !ARM
> > > > > > +    depends on ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
> > > > > > +    default y if ARCH_HAS_DEBUG_VM_PGTABLE && DEBUG_VM
> > > > > > +    help
> > > > > >
> > > > > > The default is really irrelevant as the config option can be set explicitly.
> > > > > That could also explain. Since not long time ago, it was only âdefault
> > > > > y if DEBUG_VMâ, that caused the robot saved a .config with
> > > > > DEBUG_VM_PGTABLE=y by default.
> > > > >
> > > > > Even though you changed the rule recently, it has no effect as the
> > > > > robot could âmake oldconfigâ from the saved config for each linux-next
> > > > > tree execution and the breakage will go on.
> > > > I'm not entirely sure that's the case. This report still points at the
> > > > old commit fa6726c1e7 which has:
> > > >
> > > > +       depends on ARCH_HAS_DEBUG_VM_PGTABLE || EXPERT
> > > > +       default n if !ARCH_HAS_DEBUG_VM_PGTABLE
> > > > +       default y if DEBUG_VM
> > > >
> > > > In -next we now have commit 647d9a0de34c and subsequently modified by
> > > > commit 0a8646638865. So hopefully with the latest -next tree we won't
> > > > see this report.
> > > Could some one from LKP test framework, please confirm if this still causes
> > > above problem on the latest linux-next by default ?
> >
> > The .config is a rand config, the problem is still exist if run "make
> > oldconfig" for the config with commit 0a8646638865.
>
> Is randconfig expected to boot? I don't think it is but I guess it
> should not trigger a BUG_ON during boot.
>
> > $ grep -e CONFIG_MMU= -e CONFIG_EXPERT= -e CONFIG_ARCH_HAS_DEBUG_VM_PGTABLE=
> > -e CONFIG_DEBUG_VM= .config
> > CONFIG_EXPERT=y
> > CONFIG_MMU=y
> > CONFIG_DEBUG_VM=y
> >
> > should we disable DEBUG_VM_PGTABLE by default?
>
> If that's the only case where this fails in LKP, I'd rather remove the
> EXPERT dependency so that it cannot be enabled. Architectures that want
> to experiment with this feature will have to select
> ARCH_HAS_DEBUG_VM_PGTABLE explicitly.

But when something is not selectable, people won't even know it exists.

Why not try and fix the problems reported by the robots instead ?