[regression, 4.9, pmem] memmap= command line, pmem device creation behaviour changed

From: Dave Chinner
Date: Tue Oct 11 2016 - 19:32:16 EST

Hi Dan,

I boot my DAX test machine with "memmap=8G!16G,8G!24G" on the kernel
command line to give me two 8GB pmem devices. This has worked fine
on all kernels including 4.8. I just updated that test machine to a
TOT linus kernel (4.9), and now I get a single 16GB pmem device.
i.e. the memory map the kernel generates is different. This is
what I get on boot from a 4.9 kernel:

[ 0.000000] e820: BIOS-provided physical RAM map:
[ 0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
[ 0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000bffdefff] usable
[ 0.000000] BIOS-e820: [mem 0x00000000bffdf000-0x00000000bfffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000feffc000-0x00000000feffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
[ 0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000083fffffff] usable
[ 0.000000] NX (Execute Disable) protection: active
[ 0.000000] e820: user-defined physical RAM map:
[ 0.000000] user: [mem 0x0000000000000000-0x000000000009fbff] usable
[ 0.000000] user: [mem 0x000000000009fc00-0x000000000009ffff] reserved
[ 0.000000] user: [mem 0x00000000000f0000-0x00000000000fffff] reserved
[ 0.000000] user: [mem 0x0000000000100000-0x00000000bffdefff] usable
[ 0.000000] user: [mem 0x00000000bffdf000-0x00000000bfffffff] reserved
[ 0.000000] user: [mem 0x00000000feffc000-0x00000000feffffff] reserved
[ 0.000000] user: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
[ 0.000000] user: [mem 0x0000000100000000-0x00000003ffffffff] usable
[ 0.000000] user: [mem 0x0000000400000000-0x00000007ffffffff] persistent (type 12)
[ 0.000000] user: [mem 0x0000000800000000-0x000000083fffffff] usable

On 4.8, I get two persistent (type 12) sections, each of 8GB. 4.9 is
giving me a single 16GB region. This needs to behave like a 4.8
kernel and return two persistent regions - persistent memory device
setup cannot be allowed to change from kernel to kernel. Change in
mapping and device setup like this will cause the corruption of
and/or loss of data in the persistent memory devices that have
changed shape, size or disappeared....


Dave Chinner