Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range | Date | Wed, 24 Mar 2021 15:05:54 +0100 |
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On 24.03.21 14:40, Michal Hocko wrote: > On Wed 24-03-21 14:13:31, David Hildenbrand wrote: >> On 24.03.21 13:37, Michal Hocko wrote: >>> On Wed 24-03-21 13:23:47, David Hildenbrand wrote: >>>> On 24.03.21 13:10, Michal Hocko wrote: >>>>> On Wed 24-03-21 13:03:29, Michal Hocko wrote: >>>>>> On Wed 24-03-21 11:12:59, Oscar Salvador wrote: >>>>> [...] >>>>> >>>>> an additional remark >>>>> >>>>>>> - online_pages()->move_pfn_range_to_zone(): Accounts for node/zone's spanned pages >>>>>>> - online_pages()->zone->present_pages += nr_pages; >>>>> >>>>> I am pretty sure you shouldn't account vmmemmap pages to the target zone >>>>> in some cases - e.g. vmemmap cannot be part of the movable zone, can it? >>>>> So this would be yet another special casing. This patch has got it wrong >>>>> unless I have missed some special casing. >>>>> >>>> >>>> It's a bit unfortunate that we have to discuss the very basic design >>>> decisions again. >>> >>> It would be great to have those basic design decisions layed out in the >>> changelog. >>> >>>> @Oscar, maybe you can share the links where we discussed all this and add >>>> some of it to the patch description. >>>> >>>> I think what we have right here is good enough for an initial version, from >>>> where on we can improve things without having to modify calling code. >>> >>> I have to say I really dislike vmemmap proliferation into >>> {on,off}lining. It just doesn't belong there from a layering POV. All >>> this code should care about is to hand over pages to the allocator and >>> make them visible. >> >> Well, someone has to initialize the vmemmap of the vmemmap pages ( which is >> itself :) ), > > Yeah, and I would expect this to be done when the vmemmap space is > reserved. This is at the hotadd time and we do not know the zone but > that shouldn't really matter because their zone can be quite arbitrary > kernel zone. As mentioned previously I do not think associating those > with zone movable is a good idea as they are fundamentally not movable.
I don't think that's an issue. Just another special case to keep things simple. (and not completely fragment zones, mess with zone shrinking etc.)
> It is likely that the zone doesn't really matter for these pages anyway > and the only think we do care about is that they are not poisoned and > there is at least something but again it would be much better to have a > single place where all those details are done (including accounting) > rather than trying to wrap head around different pfns when onlining > pages and grow potential and suble bugs there.
Exactly, as you said, the zone doesn't really matter - thus, this patch just handles it as simple as possible: keep them in the same zone as the hole memory block. No fragmented zones. no special casing. simple.
Details are actually pretty much all at a single place when onlining/offlining().
> >> and as the vemmap does not span complete sections things can >> get very weird as we can only set whole sections online (there was more to >> that, I think it's buried in previous discussions). > > Yes the section can only online as whole. This is an important "detail" > and it would deserve some more clarification in the changelog as well.
Indeed.
> You have invested quite some energy into code consolidation and checks > to make sure that hotplugged code doesn't have holes and this work bends > those rules. vmemmap is effectivelly a hole in a memblock/section. I > think we should re-evaluate some of those constrains rather than try to > work them around somehow. It's an offset in the beginning, so it's a special case. And the question is if there is a real benefit in handling it differently, for example, messing with online sections, messing with zones .. I am not convinced that the added complexity gives us a real benefit. But I shall be taught otherwise.
BTW: I once thought about having online_memory_block(block)/offline_memory_block(block) as separate functions instead of having pretty generic (error prone?) online_pages()/offline_pages(). Then, these details would just go in there and memory blocks + online/offline logic would simply be self-contained.
-- Thanks,
David / dhildenb
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