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

From: Georgi Djakov
Date: Mon Sep 27 2021 - 16:00:33 EST

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
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.


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