[Xen-devel] Duplicated memory nodes cause the BUG()

[Andrii Anisov <andrii_anisov@xxxxxxxx>]

Dear All,


In case of memory nodes duplication (or overlapping) in a device tree, 
we get a BUG at page_alloc.c:226.
This appears e.g. for Renesas R-Car Gen3 SoC based boards. Their u-boot 
does write memory nodes to the device tree before image (Linux kernel, 
xen) boot. So having following memory description in the compiled device 
tree for Salvator-X:
\ {
        memory@48000000 {
                device_type = "memory";
                reg = <0x0 0x48000000 0x0 0x38000000>;
        };
        memory@500000000 {
                device_type = "memory";
                reg = <0x5 0x00000000 0x0 0x40000000>;
        };
        memory@600000000 {
                device_type = "memory";
                reg = <0x6 0x00000000 0x0 0x40000000>;
        };
        memory@700000000 {
                device_type = "memory";
                reg = <0x7 0x00000000 0x0 0x40000000>;
        };
}

The hypervisor in runtime receives following:
\ {
        memory@48000000 {
                device_type = "memory";
                reg = <0x0 0x48000000 0x0 0x38000000>,
                          <0x5 0x00000000 0x0 0x40000000>,
                          <0x6 0x00000000 0x0 0x40000000>,
                          <0x7 0x00000000 0x0 0x40000000>;
        };
        memory@500000000 {
                device_type = "memory";
                reg = <0x5 0x00000000 0x0 0x40000000>;
        };
        memory@600000000 {
                device_type = "memory";
                reg = <0x6 0x00000000 0x0 0x40000000>;
        };
        memory@700000000 {
                device_type = "memory";
                reg = <0x7 0x00000000 0x0 0x40000000>;
        };
}

And crashes:

(XEN) Checking for initrd in /chosen
(XEN) RAM: 0000000048000000 - 000000007fffffff
(XEN) RAM: 0000000500000000 - 000000053fffffff
(XEN) RAM: 0000000600000000 - 000000063fffffff
(XEN) RAM: 0000000700000000 - 000000073fffffff
(XEN) RAM: 0000000500000000 - 000000053fffffff
(XEN) RAM: 0000000600000000 - 000000063fffffff
(XEN) RAM: 0000000700000000 - 000000073fffffff
(XEN)
(XEN) MODULE[0]: 0000000048000000 - 0000000048011000 Device Tree
(XEN) MODULE[1]: 000000007a000000 - 000000007c000000 Kernel
(XEN)  RESVD[0]: 0000000048000000 - 0000000048011000
(XEN)
(XEN) Command line: dom0_mem=752M console=dtuart dtuart=serial0 
dom0_max_vcpus=4
(XEN) Placing Xen at 0x000000073fe00000-0x0000000740000000
(XEN) Update BOOTMOD_XEN from 0000000078080000-0000000078192d81 => 
000000073fe00000-000000073ff12d81
(XEN) PFN compression on bits 19...19
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Boot BUG at page_alloc.c:226
(XEN) ****************************************
(XEN)
(XEN) Reboot in five seconds...

This mainly happens because duplicated memory nodes are accounted as 
separate memory banks (in process_memory_node()).
I select now of two approaches to fix the issue:
* ignore next duplicating (overlapping) memory node in favor of one 
already in a memory banks list
* merge duplicating (overlapping), even neighboring, memory banks

Actually I tend to follow the second approach, but want to get some 
advisory from community first.
ps. Linux kernel does tolerate duplicated memory nodes by merging memory 
blocks. I.e. memblock_add_range() function is commented as following:
    /**
     * memblock_add_range - add new memblock region
     * @type: memblock type to add new region into
     * @base: base address of the new region
     * @size: size of the new region
     * @nid: nid of the new region
     * @flags: flags of the new region
     *
     * Add new memblock region [@base,@base+@size) into @type.  The new 
region
     * is allowed to overlap with existing ones - overlaps don't affect 
already
     * existing regions.  @type is guaranteed to be minimal (all 
neighbouring
     * compatible regions are merged) after the addition.
     *
     * RETURNS:
     * 0 on success, -errno on failure.
     */

--

*Andrii Anisov*



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