Re: [PATCH -V2] /proc/PID/smaps: Add PMD migration entry parsing

[Konstantin Khlebnikov]

On 02/04/2020 11.21, Michal Hocko wrote:
> On Thu 02-04-20 16:10:29, Huang, Ying wrote:
> > Michal Hocko <mhocko@xxxxxxxxxx> writes:
> >
> > > On Thu 02-04-20 15:03:23, Huang, Ying wrote:
> [...]
> > > > > Could you explain why do we need this WARN_ON? I haven't really checked
> > > > > the swap support for THP but cannot we have normal swap pmd entries?
> > > >
> > > > I have some patches to add the swap pmd entry support, but they haven't
> > > > been merged yet.
> > > >
> > > > Similar checks are for all THP migration code paths, so I follow the
> > > > same style.
> > >
> > > I haven't checked other migration code paths but what is the reason to
> > > add the warning here? Even if this shouldn't happen, smaps is perfectly
> > > fine to ignore that situation, no?
> >
> > Yes. smaps itself is perfectly fine to ignore it.  I think this is used
> > to find bugs in other code paths such as THP migration related.
>
> Please do not add new warnings without a good an strong reasons. As a
> matter of fact there are people running with panic_on_warn and each
> warning is fatal for them. Please also note that this is a user trigable
> path and that requires even more care.

But this should not happen and if it does we'll never know without debug.
VM_WARN_ON checks something only if build with CONFIG_DEBUG_VM=y.

Anybody who runs debug kernels with panic_on_warn shouldn't expect much stability =)