On Fri, Nov 13, 2020 at 03:59:32PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> Stefan Agner reported a bug when using zsram on 32-bit Arm machines
> with RAM above the 4GB address boundary:
>
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> pgd = a27bd01c
> [00000000] *pgd=236a0003, *pmd=1ffa64003
> Internal error: Oops: 207 [#1] SMP ARM
> Modules linked in: mdio_bcm_unimac(+) brcmfmac cfg80211 brcmutil raspberrypi_hwmon hci_uart crc32_arm_ce bcm2711_thermal phy_generic genet
> CPU: 0 PID: 123 Comm: mkfs.ext4 Not tainted 5.9.6 #1
> Hardware name: BCM2711
> PC is at zs_map_object+0x94/0x338
> LR is at zram_bvec_rw.constprop.0+0x330/0xa64
> pc : [<c0602b38>] lr : [<c0bda6a0>] psr: 60000013
> sp : e376bbe0 ip : 00000000 fp : c1e2921c
> r10: 00000002 r9 : c1dda730 r8 : 00000000
> r7 : e8ff7a00 r6 : 00000000 r5 : 02f9ffa0 r4 : e3710000
> r3 : 000fdffe r2 : c1e0ce80 r1 : ebf979a0 r0 : 00000000
> Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
> Control: 30c5383d Table: 235c2a80 DAC: fffffffd
> Process mkfs.ext4 (pid: 123, stack limit = 0x495a22e6)
> Stack: (0xe376bbe0 to 0xe376c000)
>
> As it turns out, zsram needs to know the maximum memory size, which
> is defined in MAX_PHYSMEM_BITS when CONFIG_SPARSEMEM is set, or in
> MAX_POSSIBLE_PHYSMEM_BITS on the x86 architecture.
>
> The same problem will be hit on all 32-bit architectures that have a
> physical address space larger than 4GB and happen to not enable sparsemem
> and include asm/sparsemem.h from asm/pgtable.h.
>
> After the initial discussion, I suggested just always defining
> MAX_POSSIBLE_PHYSMEM_BITS whenever CONFIG_PHYS_ADDR_T_64BIT is
> set, or provoking a build error otherwise. This addresses all
> configurations that can currently have this runtime bug, but
> leaves all other configurations unchanged.
>
> I looked up the possible number of bits in source code and
> datasheets, here is what I found:
>
> - on ARC, CONFIG_ARC_HAS_PAE40 controls whether 32 or 40 bits are used
> - on ARM, CONFIG_LPAE enables 40 bit addressing, without it we never
> support more than 32 bits, even though supersections in theory allow
> up to 40 bits as well.
> - on MIPS, some MIPS32r1 or later chips support 36 bits, and MIPS32r5
> XPA supports up to 60 bits in theory, but 40 bits are more than
> anyone will ever ship
> - On PowerPC, there are three different implementations of 36 bit
> addressing, but 32-bit is used without CONFIG_PTE_64BIT
> - On RISC-V, the normal page table format can support 34 bit
> addressing. There is no highmem support on RISC-V, so anything
> above 2GB is unused, but it might be useful to eventually support
> CONFIG_ZRAM for high pages.
>
> Fixes: 61989a80fb3a ("staging: zsmalloc: zsmalloc memory allocation library")
> Fixes: 02390b87a945 ("mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS")
> Cc: Stefan Agner <stefan@xxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Vineet Gupta <vgupta@xxxxxxxxxxxx>
> Cc: linux-snps-arc@xxxxxxxxxxxxxxxxxxx
> Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
> Cc: linux-mips@xxxxxxxxxxxxxxx
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> Cc: Albert Ou <aou@xxxxxxxxxxxxxxxxx>
> Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/linux-mm/bdfa44bf1c570b05d6c70898e2bbb0acf234ecdf.1604762181.git.stefan@xxxxxxxx/
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> If everyone is happy with this version, I would suggest merging this as
> a bugfix through my asm-generic tree for linux-5.10. I originally
> said I'd send individual patches for each architecture tree, but
> I now think this is easier and better documents what is going on.
> ---
> arch/mips/include/asm/pgtable-32.h | 3 +++
Acked-by: Thomas Bogendoerfer <tsbogend@xxxxxxxxxxxxxxxx>
Thomas.
--
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea. [ RFC1925, 2.3 ]
