Hi Andrea, Maciej,

On 02/06/2017 03:17 AM, Andrea Reale wrote:

Hi Scott,
Hi all,

in reply to the issues that Scott reported last month, myself and Maciej
investigated further by running quite a number of experiments on the
physical and virtual environments we have avaialable.

We collected all the results and relevant logs in a Web page at
https://hotplug-tests.eu-gb.mybluemix.net/ so that anyone interested can
go there and check all the details.

The tl;dr version is that, in all configuration, we could not reproduce
what Scott has described as "memory corruption". The only issue we
encountered happens when the system is booted with a small amount of
initial memory (e.g., mem=64M) and one tries to hot-add several sections
of memory in ZONE_MOVABLE; in that case, the process is likely to fail
when vmemmap tries to allocate chunks of 2^9 consecutive pages to make
space for the `struct page`s describing the new memory; in fact, it
seems likely that, in low memory situations, the system cannot find enough
consecutive pages in ZONE_DMA or ZONE_NORMAL. This condition is not
dependand on memory hot-plug; in fact, we counter-tested this by writing
a simple module that just tries to allocate a few chunks of 2^9 pages,
and we experienced that it fails when the system is booted with low
memory (sources and logs in the Web page linked above).

@Scott: were your referring to this issue, by any chance, in your
previous emails? If not, we would really appreciate if you could help us
reproduce the condition you are experiencing and/or give us a more detail 
of what are the symptoms of the corruption you are referring to.

One question regarding your patch posted here:
https://lkml.org/lkml/2016/12/14/188

While the "hack" that sets/clears NOMAP in order for pfn_valid() to
return false/true when appropriate during __add_pages() definitively
does seem to work to probe the memory section, don't you also hit the
same warning when you try to online that memory section in
pages_correctly_reserved() once you have cleared the NOMAP flag?

NB: I am working on the 4.1 kernel at the moment, but it seems to be
nearly identical in that regard.

We are still running additional tests on other boards and we will update
the Web page while we get them. If anyone happens to try these patches
on their system, we warmly invite to send feedback with either 
negative or positive outcomes.

I will definitively give this a try on ARM64 since I need to get it
working there.

Do you mind posting a non-RFC patch?

Thanks!
--
Florian