Re: CONFIG_DEBUG_VIRTUAL breaks boot on x86-32

[William Kucharski]

The dmesg output you posted confirms that max_low_pfn is indeed 0x373fe, and it appears
that the value of phys_mem being checked mat be 0x3f401ff1, which translates to pfn 0x3f401,
at least if what's still in registers can be believed.

Since that is indeed greater than max_low_pfn, VIRTUAL_BUG_ON triggers:

    VIRTUAL_BUG_ON((phys_addr >> PAGE_SHIFT) > max_low_pfn);

Looking at the call stack of

    copy_strings_0x220
        __check_object_size+0xef

that looks to translate to this sequence:

copy_from_user(kaddr+offset, str, bytes_to_copy)
    check_copy_size(kaddr+offset, bytes_to_copy, FALSE)
        check_object_size(kaddr+offset, bytes_to_copy, FALSE)
            __check_object_size(kaddr+offset, bytes_to_copy, FALSE)
                check_heap_object(kaddr+offset, bytes_to_copy, FALSE)
                    virt_to_head_page(kaddr+offset)
                        virt_to_page(kaddr+offset)
                            pfn_to_page(kaddr+offset)
                                __pa(kaddr+offset)
                                    __phys_addr(kaddr+offset)

so it appears the address is "too high" for low memory at that point.