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

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 02/04/2020 11.21, Michal Hocko wrote:
> On Thu 02-04-20 16:10:29, Huang, Ying wrote:
>> Michal Hocko <mhocko@kernel.org> 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 =)