Memory size unaligned to section boundary

[Stefan Agner]

Hi,

It seems to me that I hit an issue in low memory mapping (map_lowmem).
I'm using a custom memory size, which leads to an freeze on Linux 4.0
and also with Linus master on two tested ARMv7-A SoC's (Freescale Vybrid
and NVIDIA Tegra 3):

With mem=259744K
[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 4.0.0-00189-ga4d2a4c3-dirty
(ags at trochilidae) (gcc version 4.8.3 20140401 (prerelease) (Linaro GCC
4.8-2014.04) ) #506 Thu Apr 23 14:13:21 CEST 2015
[    0.000000] CPU: ARMv7 Processor [410fc051] revision 1 (ARMv7),
cr=10c5387d
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing
instruction cache
[    0.000000] Machine model: Toradex Colibri VF61 on Colibri Evaluation
Board
[    0.000000] bootconsole [earlycon0] enabled
[    0.000000] cma: Reserved 16 MiB at 0x8e400000
[    0.000000] Memory policy: Data cache writeback
<freeze>

I dug a bit more into that, and it unveiled that when creating the
mapping for the non-kernel_x part (if (kernel_x_end < end) in
map_lowmem), the unaligned section at the end leads to the freeze. In
alloc_init_pmd, if the memory end is section unaligned, alloc_init_pte
gets called which allocates a PTE outside of the initialized region (in
early_alloc_aligned). The system freezes at the call of memset in
early_alloc_aligned function.

With some debug print, this can be better illustrated:
[    0.000000] pgd 800063f0, addr 8fc00000, end 8fda8000, next 8fda8000
[    0.000000] pud 800063f0, addr 8fc00000, end 8fda8000, next 8fda8000
[    0.000000] pmd 800063f0, addr 8fc00000, next 8fda8000
                         => actual end of memory ^^^^^^^^
[    0.000000] alloc_init_pte
[    0.000000] set_pte_ext, pte 00000000, addr 8fc00000, end 8fda8000
[    0.000000] early_pte_alloc
[    0.000000] early_alloc_aligned, 00001000, ptr 8fcff000, align
00001000
     => PTE allocated outside of initialized area ^^^^^^^^

It seems that memory gets allocation in the last section. When the last
section was in the previous PMD, the allocation works, however if the
last section is within the same PMD, the allocation ends up in the
non-initialized area. So:

In other words, sizes which end in a upper part of the 2MB sized PMD
fail, while sizes in the lower part of a PMD work.
0xFF80000 => fails (mem=261632K)
0xFE80000 => works (mem=260608K)
0xFD80000 => fails (mem=261632K)
...

While I understand the reason for the freeze, I don't know to properly
fix it. It looks to me that in alloc_init_pmd, we should use
__map_init_section first to map the last aligned section, before calling
alloc_init_pte on the non aligned section.

Background: I tried to reuse the boot loader part of the simplefb
implementation for sunxi. It decreases memory size by the size of the
framebuffer. Hence the actually memory size can be unaligned, depending
on the display size used. In the case at hand, a framebuffer of the size
800x600 worked while 1024x600 did not work... The implementation uses
device tree to report the memory size, but the kernel arguments show the
same behavior.

Maybe a regression of e651eab0af ("ARM: 7677/1: LPAE: Fix mapping in
alloc_init_section for unaligned addresses"). I currently do not have a
platform at hand which works on that Linux version out of the box.

--
Stefan