Re: [PATCH v2 1/2] mm: Move page struct poisoning to CONFIG_DEBUG_VM_PAGE_INIT_POISON

[Pasha Tatashin <Pavel.Tatashin@microsoft.com>]

On 9/6/18 11:41 AM, Alexander Duyck wrote:
> On Thu, Sep 6, 2018 at 8:13 AM Michal Hocko <mhocko@kernel.org> wrote:
>>
>> On Thu 06-09-18 07:59:03, Dave Hansen wrote:
>>> On 09/05/2018 10:47 PM, Michal Hocko wrote:
>>>> why do you have to keep DEBUG_VM enabled for workloads where the boot
>>>> time matters so much that few seconds matter?
>>>
>>> There are a number of distributions that run with it enabled in the
>>> default build.  Fedora, for one.  We've basically assumed for a while
>>> that we have to live with it in production environments.
>>>
>>> So, where does leave us?  I think we either need a _generic_ debug
>>> option like:
>>>
>>>       CONFIG_DEBUG_VM_SLOW_AS_HECK
>>>
>>> under which we can put this an other really slow VM debugging.  Or, we
>>> need some kind of boot-time parameter to trigger the extra checking
>>> instead of a new CONFIG option.
>>
>> I strongly suspect nobody will ever enable such a scary looking config
>> TBH. Besides I am not sure what should go under that config option.
>> Something that takes few cycles but it is called often or one time stuff
>> that takes quite a long but less than aggregated overhead of the former?
>>
>> Just consider this particular case. It basically re-adds an overhead
>> that has always been there before the struct page init optimization
>> went it. The poisoning just returns it in a different form to catch
>> potential left overs. And we would like to have as many people willing
>> to running in debug mode to test for those paths because they are
>> basically impossible to review by the code inspection. More importantnly
>> the major overhead is boot time so my question still stands. Is this
>> worth a separate config option almost nobody is going to enable?
>>
>> Enabling DEBUG_VM by Fedora and others serves us a very good testing
>> coverage and I appreciate that because it has generated some useful bug
>> reports. Those people are paying quite a lot of overhead in runtime
>> which can aggregate over time is it so much to ask about one time boot
>> overhead?
> 
> The kind of boot time add-on I saw as a result of this was about 170
> seconds, or 2 minutes and 50 seconds on a 12TB system. I spent a
> couple minutes wondering if I had built a bad kernel or not as I was
> staring at a dead console the entire time after the grub prompt since
> I hit this so early in the boot. That is the reason why I am so eager
> to slice this off and make it something separate. I could easily see
> this as something that would get in the way of other debugging that is
> going on in a system.
> 
> If we don't want to do a config option, then what about adding a
> kernel parameter to put a limit on how much memory we will initialize
> like this before we just start skipping it. We could put a default
> limit on it like 256GB and then once we cross that threshold we just
> don't bother poisoning any more memory. With that we would probably be
> able to at least cover most of the early memory init, and that value
> should cover most systems without getting into delays on the order of
> minutes.

I am OK with a boot parameter to optionally disable it when DEBUG_VM is
enabled. But, I do not think it is a good idea to make that parameter
"smart" basically always poison memory with DEBUG_VM unless bootet with
a parameter that tells not to poison memory.

CONFIG_DEBUG_VM is disbled on:

RedHat, Oracle Linux, CentOS, Ubuntu, Arch Linux, SUSE

Enabled on:

Fedora

Are there other distros where it is enabled? I think, this could be
filed as a performance bug against Fedora distro, and let the decide
what to do about it.

I do not want to make this feature less tested. Poisoning memory allowed
us to catch corner case bugs like these:

ab1e8d8960b68f54af42b6484b5950bd13a4054b
mm: don't allow deferred pages with NEED_PER_CPU_KM

e181ae0c5db9544de9c53239eb22bc012ce75033
mm: zero unavailable pages before memmap init

And several more that were fixed by other people.

For a very long linux relied on assumption that boot memory is zeroed,
and I am sure we will continue detect more bugs over time.

Thank you,
Pavel

> 
> - Alex
>