From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Wed, 6 May 2015 11:11:05 +0100 Subject: Memory size unaligned to section boundary In-Reply-To: References: Message-ID: <20150506101104.GD2067@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 > > > 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; -- FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up according to speedtest.net.