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

[Michal Hocko]

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.

-- 
Michal Hocko
SUSE Labs