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

[David Hildenbrand]

On 27.09.21 22:00, Georgi Djakov wrote:
> 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.

That is the old code. I don't see how the behavior of the old code with 
wrong max_pfn was doing what it's supposed to do.

page_idle and page_owner also rely on max_pfn. The root issue is that 
max_pfn wasn't updated properly.

--
Thanks,

David / dhildenb