From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Wed, 29 Jul 2015 13:53:24 +0100 Subject: bug in identity map for 4KB pages? In-Reply-To: References: <20150729114258.GK15213@leverpostej> Message-ID: <20150729125324.GO15213@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Jul 29, 2015 at 12:49:53PM +0100, Ard Biesheuvel wrote: > On 29 July 2015 at 13:42, Mark Rutland wrote: > > On Wed, Jul 29, 2015 at 08:47:10AM +0100, Ard Biesheuvel wrote: > >> On 29 July 2015 at 04:37, Stuart Yoder wrote: > >> > Our system has RAM at a high address, and previously required using 48-bit VA > >> > in order to have an idmap that covered all of RAM. > >> > > >> > In testing on 4.2-rc4, which now contains support for the increased VA range > >> > of the idmap without using 48-bit VA, I'm finding that things work for > >> > 64KB pages, but do not for 4KB pages. > >> > > >> > Is there any known limitation here with 4KB pages? Any ideas? > >> > > >> > >> You probably have memory at 0x8000_0000 and at 0x80_8000_0000, right? > >> So the physical arrangement still requires more than the 39 bits of > >> virtual address space you get with 3 levels, even if the ID map can > >> cope now. That is why you get the 0x40_0000_0000 virtual address: > >> __phys_to_virt() just wraps to a positive number. > > > > Ah, I see. So it's the linear mapping rather than the idmap which is the > > problem. > > > > We cut memory which would fall below the start of the linear map in > > early_init_dt_add_memory_arch, by cutting memory below phys_offset. > > > > It looks like we already have the logic for cutting memory beyond the > > end of the linear map, so we should just need to override the limit. > > > > Stuart, does the below patch prevent the panic you see? > > > > Wouldn't something like this make more sense? > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c > index 597831bdddf3..64480b65ef17 100644 > --- a/arch/arm64/mm/init.c > +++ b/arch/arm64/mm/init.c > @@ -158,6 +158,15 @@ early_param("mem", early_mem); > > void __init arm64_memblock_init(void) > { > + /* > + * Remove the memory that we will not be able to cover > + * with the linear mapping. > + */ > + const s64 linear_region_size = -(s64)PAGE_OFFSET; > + > + memblock_remove(0, memstart_addr); > + memblock_remove(memstart_addr + linear_region_size, ULLONG_MAX); The maths is more correct, certainly. I'd prefer to handle the truncation in early_init_dt_add_memory_arch because it already has the requisite logic, and keeping the truncation in one place should keep things less confusing. To that end I'll respin my patch using PAGE_OFFSET to determine the physical memory limit. Unless there's something I've missed? Thanks, Mark.