Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation

David Hildenbrand
2020-09-24T14:30:09Z

>> If that would ever change, the optimization here would be lost and we
>> would have to think of something else. Nothing would actually break -
>> and it's all kept directly in page_alloc.c
>
> Sure, but then it can become a pointless code churn.

Indeed, and if there are valid concerns that this will happen in the
near future (e.g., < 1 year), I agree that we should look into
alternatives right from the start. Otherwise it's good enough until some
of the other things I mentioned below become real (which could also take
a while ...).

>
>> I'd like to stress that what I propose here is both simple and powerful.
>>
>>> possible I think, such as preparing a larger MIGRATE_UNMOVABLE area in the
>>> existing memory before we allocate those long-term management structures. Or
>>> onlining a bunch of blocks as zone_movable first and only later convert to
>>> zone_normal in a controlled way when existing normal zone becomes depeted?
>>
>> I see the following (more or less complicated) alternatives
>>
>> 1) Having a larger MIGRATE_UNMOVABLE area
>>
>> a) Sizing it is difficult. I mean you would have to plan ahead for all
>> memory you might eventually hotplug later - and that could even be
>
> Yeah, hence my worry about existing interfaces that work on 128MB blocks
> individually without a larger strategy.

Yes, in the works :)

>
>> impossible if you hotplug quite a lot of memory to a smaller machine.
>> (I've seen people in the vm/container world trying to hotplug 128GB
>> DIMMs to 2GB VMs ... and failing for obvious reasons)
>
> Some planning should still be possible to maximize the contiguous area without
> unmovable allocations.

Indeed, optimizing that is very high on my list of things to look into ...

>>
>> we would, once again, never be able to allocate a gigantic page because
>> all [N] would contain a memmap.
>
> The second approach should work, if you know how much you are going to online,
> and plan the size the N group accordingly, and if the onlined amount is several
> gigabytes, then only the first one (or first X) will be unusable for a gigantic
> page, but the rest would be? Can't get much better than that.

Indeed, it's the optimal case (assuming one can come up with a safe zone
balance - which is usually possible, but unfortunately, there are
exceptions one at least has to identify).

[...]

>
> I've reviewed the series and I won't block it - yes it's an optimistic approach
> that can break and leave us with code churn. But at least it's not that much

Thanks.

I'll try to document somewhere that the behavior of FOP_TO_TAIL is a
pure optimization and might change in the future - along with the case
it tried to optimize (so people know what the use case was).

> code and the extra test in __free_one_page() shouldn't make this hotpath too

I assume the compiler is able to completely propagate constants and
optimize that out - I haven't checked, though.

> worse. But I still hope we can achieve a more robust solution one day.

I definitely agree. I'd also prefer some kind of guarantees, but I
learned that things always sound easier than they actually are when it
comes to memory management in Linux ... and they take a lot of time (for
example, Michal's/Oscar's attempts to implement vmemmap on hotadded memory).

--
Thanks,

David / dhildenb