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SubjectRe: [PATCH v10 05/10] mm: zero reserved and unavailable struct pages
On Fri 06-10-17 11:25:16, Pasha Tatashin wrote:
> Hi Michal,
>
> >
> > As I've said in other reply this should go in only if the scenario you
> > describe is real. I am somehow suspicious to be honest. I simply do not
> > see how those weird struct pages would be in a valid pfn range of any
> > zone.
> >
>
> There are examples of both when unavailable memory is not part of any zone,
> and where it is part of zones.
>
> I run Linux in kvm with these arguments:
>
> qemu-system-x86_64
> -enable-kvm
> -cpu kvm64
> -kernel $kernel
> -initrd $initrd
> -m 512
> -smp 2
> -device e1000,netdev=net0
> -netdev user,id=net0
> -boot order=nc
> -no-reboot
> -watchdog i6300esb
> -watchdog-action debug
> -rtc base=localtime
> -serial stdio
> -display none
> -monitor null
>
> This patch reports that there are 98 unavailable pages.
>
> They are: pfn 0 and pfns in range [159, 255].
>
> Note, trim_low_memory_range() reserves only pfns in range [0, 15], it does
> not reserve [159, 255] ones.
>
> e820__memblock_setup() reports linux that the following physical ranges are
> available:
> [1 , 158]
> [256, 130783]
>
> Notice, that exactly unavailable pfns are missing!
>
> Now, lets check what we have in zone 0: [1, 131039]
>
> pfn 0, is not part of the zone, but pfns [1, 158], are.
>
> However, the bigger problem we have if we do not initialize these struct
> pages is with memory hotplug. Because, that path operates at 2M boundaries
> (section_nr). And checks if 2M range of pages is hot removable. It starts
> with first pfn from zone, rounds it down to 2M boundary (sturct pages are
> allocated at 2M boundaries when vmemmap is created), and and checks if that
> section is hot removable. In this case start with pfn 1 and convert it down
> to pfn 0.

Hmm, this is really interesting! I thought each memblock is guaranteed
to be section size aligned. But I suspect this is more of a wishful
thinking. But now I see what is the problem.
--
Michal Hocko
SUSE Labs

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