http://linux-hacks.blogspot.com/2009/08/dumping-kernel-page-tables.htmlMonday, August 3, 2009Dumping kernel page tablesSometimes when
debugging kernel issues, you might come across kernel addresses that
you would find very difficult to map to a particular section in the
kernel, ie, vmalloc, vmemmap, low/high kernel mapping, kernel text,
etc. On x86, Arjan van de Ven has written an interface that provides a
dump of the kernel page tables which gives information on the various
memory areas in the kernel.
# cat /debug/kernel_page_tables ---[ User Space ]--- 0x0000000000000000-0xffff800000000000 16777088T pgd ---[ Kernel Space ]--- 0xffff800000000000-0xffff880000000000 8T pgd ---[ Low Kernel Mapping ]--- 0xffff880000000000-0xffff880000200000 2M RW GLB x pte 0xffff880000200000-0xffff880040000000 1022M RW PSE GLB x pmd 0xffff880040000000-0xffff8800cfe00000 2302M RW PSE GLB NX pmd ... ---[ vmalloc() Area ]--- 0xffffc20000000000-0xffffc20000001000 4K RW PCD GLB NX pte 0xffffc20000001000-0xffffc20000004000 12K pte 0xffffc20000004000-0xffffc20000005000 4K RW PCD GLB NX pte 0xffffc20000005000-0xffffc20000008000 12K pte 0xffffc20000008000-0xffffc2000000d000 20K RW PCD GLB NX pte 0xffffc2000000d000-0xffffc20000010000 12K pte 0xffffc20000010000-0xffffc20000011000 4K RW PCD GLB NX pte .... ---[ Vmemmap ]--- 0xffffe20000000000-0xffffe20007c00000 124M RW PSE GLB NX pmd 0xffffe20007c00000-0xffffe20040000000 900M pmd 0xffffe20040000000-0xffffe28000000000 511G pud 0xffffe28000000000-0xffffff8000000000 29T pgd 0xffffff8000000000-0xffffffff80000000 510G pud ---[ High Kernel Mapping ]--- 0xffffffff80000000-0xffffffff80200000 2M pmd 0xffffffff80200000-0xffffffff80a00000 8M RW PSE GLB x pmd 0xffffffff80a00000-0xffffffffa0000000 502M pmd ---[ Modules ]--- 0xffffffffa0000000-0xffffffffa000a000 40K RW GLB x pte 0xffffffffa000a000-0xffffffffa000f000 20K pte 0xffffffffa000f000-0xffffffffa0016000 28K RW GLB x pte 0xffffffffa0016000-0xffffffffa001b000 20K pte .... ... Understanding the above output: o First field indicates the address range of a particular type of area (for example, user space, vmalloc area, kernel space, etc) o The second field indicates the size of the address range in K,M,G,T units o The fields following the size of the range have the following meaning: USR - whether the page being mapped is a user page or not RW - whether the page is read/write. If not RW, the output would be 'ro' to indicate a read-only page PCD - Page Cache Disabled - maps a page with caching disabled PWT - page with Page Write-Through set PSE - Extended paging enabled - allows large linear contiguous address ranges to be mapped GLB - Page Global flag - The global flag is set for a page that is frequently used and prevents it from being flushed from the TLB NX - Page is non-executable, else marked as 'x' o The last entry indicates the particular level of the page table - pgd, pud, pmd or pte that the region corresponds to Enable the CONFIG_X86_PTDUMP configuration option, along with enabling debugfs. The corresponding kernel code for the interface can be found under arch/x86/mm/dump_pagetables.c  |
