Memory size unaligned to section boundary

On Thu, Apr 23, 2015 at 03:19:45PM +0200, Stefan Agner wrote:

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 ^^^^^^^^

Right, and the question is why does that happen - and the answer is
this:

        /*
         * Round the memblock limit down to a section size.  This
         * helps to ensure that we will allocate memory from the
         * last full section, which should be mapped.
         */
        if (memblock_limit)
                memblock_limit = round_down(memblock_limit, SECTION_SIZE);

That should round down by 2x SECTION_SIZE to ensure that we don't start
allocating the L2 page table in a section which isn't mapped.  Please
try this patch:

 arch/arm/mm/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 4e6ef896c619..387becac5c86 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c

@@ -1142,7 +1142,7 @@ void __init sanity_check_meminfo(void)
 	 * last full section, which should be mapped.
 	 */
 	if (memblock_limit)
-		memblock_limit = round_down(memblock_limit, SECTION_SIZE);
+		memblock_limit = round_down(memblock_limit, 2 * SECTION_SIZE);
 	if (!memblock_limit)
 		memblock_limit = arm_lowmem_limit;
 

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