Re: [RFC] arm64: mm: update max_pfn after memory hotplug

[Georgi Djakov]

On 9/27/2021 8:34 PM, David Hildenbrand wrote:
> On 27.09.21 19:22, Georgi Djakov wrote:
> > On 9/24/2021 1:54 AM, Chris Goldsworthy wrote:
> > > From: Sudarshan Rajagopalan <quic_sudaraja@xxxxxxxxxxx>
> > >
> > > After new memory blocks have been hotplugged, max_pfn and max_low_pfn
> > > needs updating to reflect on new PFNs being hot added to system.
> > >
> > > Signed-off-by: Sudarshan Rajagopalan <quic_sudaraja@xxxxxxxxxxx>
> > > Signed-off-by: Chris Goldsworthy <quic_cgoldswo@xxxxxxxxxxx>
> >
> > Thanks for the patch, Chris!
> >
> > With this patch, the data in /proc/kpageflags appears to be correct and
> > memory tools like procrank work again on arm64 platforms.
> >
> > Tested-by: Georgi Djakov <quic_c_gdjako@xxxxxxxxxxx>
> >
> > Maybe we should add fixes tag, as it has been broken since the following
> > commit:
> > Fixes: abec749facff ("fs/proc/page.c: allow inspection of last section
> > and fix end detection")
>
> Are you sure that that commit broke it?

Reverting the above commit also "fixes" kpageflags, otherwise
kpageflags_read() returns 0 in the following check:
	if (src >= max_dump_pfn * KPMSIZE)
		return 0;

> I recall that we would naturally run into the limit, because
>
> count = min_t(size_t, count, (max_pfn * KPMSIZE) - src);

The function returns before we reach this line.

Thanks,
Georgi

> wouldn't really do what you would expect either. But you could 
> force-read beyond max_pfn, yes, because the count computation was just 
> weird.
>
>
> I think the real issue is not properly adjusting max_pfn in the first 
> place when we introduced memoruy hotplug on arm64