Subject: Re: [PATCH 1/2] powerpc/pagetable: Add option to dump the linux pagetables

From: Balbir Singh <[email-masked]>
Date: Wed, 30 Mar 2016 14:24:25 +1100

On 22/03/16 09:04, Rashmica Gupta wrote:
> Useful to be able to dump the kernels page tables to check permissions
> and memory types - derived from arm64's implementation.
>
> Add a debugfs file to check the page tables. To use this the PPC_PTDUMP
> config option must be selected.
>
> Signed-off-by: Rashmica Gupta <[email-masked]>
> ---
>  arch/powerpc/Kconfig.debug             |  12 ++
>  arch/powerpc/mm/Makefile               |   1 +
>  arch/powerpc/mm/dump_linuxpagetables.c | 377 +++++++++++++++++++++++++++++++++
>  3 files changed, 390 insertions(+)
>  create mode 100644 arch/powerpc/mm/dump_linuxpagetables.c
>
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index 638f9ce740f5..26a60effea1a 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -344,4 +344,16 @@ config FAIL_IOMMU
>  
>  	  If you are unsure, say N.
>  
> +config PPC_PTDUMP
> +        bool "Export kernel pagetable layout to userspace via debugfs"
> +        depends on DEBUG_KERNEL
> +        select DEBUG_FS
> +        help
> +	  This option exports the state of the kernel pagetables to a
> +	  debugfs file. This is only useful for kernel developers who are
> +	  working in architecture specific areas of the kernel - probably
> +	  not a good idea to enable this feature in a production kernel.
> +
> +	  If you are unsure, say N.
> +

Some minor comments below, but otherwise

Acked-by: Balbir Singh <[email-masked]>

>  endmenu
> diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> index adfee3f1aeb9..6935c6204fbc 100644
> --- a/arch/powerpc/mm/Makefile
> +++ b/arch/powerpc/mm/Makefile
> @@ -41,3 +41,4 @@ obj-$(CONFIG_NOT_COHERENT_CACHE) += dma-noncoherent.o
>  obj-$(CONFIG_HIGHMEM)		+= highmem.o
>  obj-$(CONFIG_PPC_COPRO_BASE)	+= copro_fault.o
>  obj-$(CONFIG_SPAPR_TCE_IOMMU)	+= mmu_context_iommu.o
> +obj-$(CONFIG_PPC_PTDUMP)	+= dump_linuxpagetables.o
> diff --git a/arch/powerpc/mm/dump_linuxpagetables.c b/arch/powerpc/mm/dump_linuxpagetables.c
> new file mode 100644
> index 000000000000..f97fbfdac4b9
> --- /dev/null
> +++ b/arch/powerpc/mm/dump_linuxpagetables.c
> @@ -0,0 +1,377 @@
> +/*
> + * Copyright 2016, Rashmica Gupta, IBM Corp.
> + *
> + * This traverses the kernel pagetables and dumps the
> + * information about the used sections of memory to
> + * /sys/kernel/debug/kernel_pagetables.
> + *
> + * Derived from the arm64 implementation:
> + * Copyright (c) 2014, The Linux Foundation, Laura Abbott.
> + * (C) Copyright 2008 Intel Corporation, Arjan van de Ven.
> + *
> + * This program is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU General Public License
> + * as published by the Free Software Foundation; version 2
> + * of the License.
> + */
> +#include <linux/debugfs.h>
> +#include <linux/fs.h>
> +#include <linux/io.h>
> +#include <linux/mm.h>
> +#include <linux/sched.h>
> +#include <linux/seq_file.h>
> +#include <asm/fixmap.h>
> +#include <asm/pgtable.h>
> +#include <linux/const.h>
> +#include <asm/page.h>
> +#include <asm/pgalloc.h>
> +
> +struct addr_marker {
> +	unsigned long start_address;
> +	const char *name;
> +};
> +
> +static struct addr_marker address_markers[] = {
> +	{ VMALLOC_START,	"vmalloc() Area" },
> +	{ VMALLOC_END,		"vmalloc() End" },
> +	{ ISA_IO_BASE,		"isa I/O start" },
> +	{ ISA_IO_END,		"isa I/O end" },
> +	{ PHB_IO_BASE,		"phb I/O start" },
> +	{ PHB_IO_END,		"phb I/O end" },
> +	{ IOREMAP_BASE,		"I/O remap start" },
> +	{ IOREMAP_END,		"I/O remap end" },
> +	{ -1,			NULL },
> +};
> +
> +/*
> + * To visualise what is happening,
> + *
> + *  - PTRS_PER_P** = how many entries there are in the corresponding P**
> + *  - P**_SHIFT = how many bits of the address we use to index into the
> + * corresponding P**
> + *  - P**_SIZE is how much memory we can access through the table - not the
> + * size of the table itself.
> + * P**={PGD, PUD, PMD, PTE}
> + *
> + *
> + * Each entry of the PGD points to a PUD. Each entry of a PUD points to a
> + * PMD. Each entry of a PMD points to a PTE. And every PTE entry points to
> + * a page.
> + *
> + * In the case where there are only 3 levels, the PUD is folded into the
> + * PGD: every PUD has only one entry which points to the PMD.
> + *
> + * The page dumper groups page table entries of the same type into a single
> + * description. It uses pg_state to track the range information while
> + * iterating over the PTE entries. When the continuity is broken it then
> + * dumps out a description of the range - ie PTEs that are virtually contiguous
> + * with the same PTE flags are chunked together. This is to make it clear how
> + * different areas of the kernel virtual memory are used.
> + *
> + */
> +struct pg_state {
> +	struct seq_file *seq;
> +	const struct addr_marker *marker;
> +	unsigned long start_address;
> +	unsigned level;
> +	u64 current_flags;
> +};
> +
> +struct flag_info {
> +	u64		mask;
> +	u64		val;
> +	const char	*set;
> +	const char	*clear;
> +};
> +
> +static const struct flag_info flag_array[] = {
> +	{
> +		.mask	= _PAGE_USER,
> +		.val	= _PAGE_USER,
> +		.set	= "user",
> +		.clear	= "    ",
> +	}, {
> +		.mask	= _PAGE_RW,
> +		.val	= _PAGE_RW,
> +		.set	= "rw",
> +		.clear	= "ro",
> +	}, {
> +		.mask	= _PAGE_EXEC,
> +		.val	= _PAGE_EXEC,
> +		.set	= " X ",
Can we make this small "x" to sort of go in line with "rw"/"ro"
> +		.clear	= "   ",
> +	}, {
> +		.mask	= _PAGE_PTE,
> +		.val	= _PAGE_PTE,
> +		.set	= "pte",
> +		.clear	= "   ",
> +	}, {
> +		.mask	= _PAGE_PRESENT,
> +		.val	= _PAGE_PRESENT,
> +		.set	= "present",
> +		.clear	= "       ",
> +	}, {
> +		.mask	= _PAGE_HASHPTE,
> +		.val	= _PAGE_HASHPTE,
> +		.set	= "hpte",
> +		.clear	= "    ",
> +	}, {
> +		.mask	= _PAGE_GUARDED,
> +		.val	= _PAGE_GUARDED,
> +		.set	= "guarded",
> +		.clear	= "       ",
> +	}, {
> +		.mask	= _PAGE_DIRTY,
> +		.val	= _PAGE_DIRTY,
> +		.set	= "dirty",
> +		.clear	= "     ",
> +	}, {
> +		.mask	= _PAGE_ACCESSED,
> +		.val	= _PAGE_ACCESSED,
> +		.set	= "accessed",
> +		.clear	= "        ",
> +	}, {
> +		.mask	= _PAGE_WRITETHRU,
> +		.val	= _PAGE_WRITETHRU,
> +		.set	= "write through",
> +		.clear	= "             ",
> +	}, {
> +		.mask	= _PAGE_NO_CACHE,
> +		.val	= _PAGE_NO_CACHE,
> +		.set	= "no cache",
> +		.clear	= "        ",
> +	}, {
> +		.mask	= _PAGE_BUSY,
> +		.val	= _PAGE_BUSY,
> +		.set	= "busy",
> +	}, {
> +		.mask	= _PAGE_COMBO,
> +		.val	= _PAGE_COMBO,
> +		.set	= "combo",
> +	}, {
> +		.mask	= _PAGE_F_GIX,
> +		.val	= _PAGE_F_GIX,
> +		.set	= "f_gix",
Can we use "primary"?
> +	}, {
> +		.mask	= _PAGE_F_SECOND,
> +		.val	= _PAGE_F_SECOND,
> +		.set	= "f_second",
"secondary idx"
> +	}, {
> +		.mask	= _PAGE_SPECIAL,
> +		.val	= _PAGE_SPECIAL,
> +		.set	= "special",
> +	}, {
> +		.mask	= _PAGE_4K_PFN,
> +		.val	= _PAGE_4K_PFN,
> +		.set	= "4K_pfn"
> +	}
> +};
> +
> +struct pgtable_level {
> +	const struct flag_info *flag;
> +	size_t num;
> +	u64 mask;
> +};
> +
> +static struct pgtable_level pg_level[] = {
> +	{
> +	}, { /* pgd */
> +		.flag	= flag_array,
> +		.num	= ARRAY_SIZE(flag_array),
> +	}, { /* pud */
> +		.flag	= flag_array,
> +		.num	= ARRAY_SIZE(flag_array),
> +	}, { /* pmd */
> +		.flag	= flag_array,
> +		.num	= ARRAY_SIZE(flag_array),
> +	}, { /* pte */
> +		.flag	= flag_array,
> +		.num	= ARRAY_SIZE(flag_array),
> +	},
> +};
> +
> +static void dump_flag(struct pg_state *st, const struct flag_info *flag,
> +			size_t num)
> +{
> +	unsigned i;
> +
> +	for (i = 0; i < num; i++, flag++) {
> +		const char *s;
> +
> +		if ((st->current_flags & flag->mask) == flag->val)
> +			s = flag->set;
> +		else
> +			s = flag->clear;
> +
> +		if (s)
> +			seq_printf(st->seq, " %s", s);
> +	}
> +}
> +
> +static void dump_addr(struct pg_state *st, unsigned long addr)
> +{
> +	static const char units[] = "KMGTPE";
> +	const char *unit = units;
> +	unsigned long delta;
> +
> +	seq_printf(st->seq, "0x%016lx-0x%016lx   ", st->start_address, addr-1);
> +	delta = (addr - st->start_address) >> 10;
> +	/* Work out what appropriate unit to use */
> +	while (!(delta & 1023) && unit[1]) {

1023, I think ((1 << 10) - 1) is easier to understand but again you may find a better way of encapsulating this

> +		delta >>= 10;
> +		unit++;
> +	}
> +	seq_printf(st->seq, "%9lu%c", delta, *unit);
> +
> +}
> +
> +static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
> +				u64 val)
> +{
> +	u64 flag = val & pg_level[level].mask;
> +
> +	/* At first no level is set */
> +	if (!st->level) {
> +		st->level = level;
> +		st->current_flags = flag;
> +		st->start_address = addr;
> +		seq_printf(st->seq, "---[ %s ]---\n", st->marker->name);
> +	/*
> +	 * Dump the section of virtual memory when:
> +	 *   - the PTE flags from one entry to the next differs.
> +	 *   - we change levels in the tree.
> +	 *   - the address is in a different section of memory and is thus
> +	 *   used for a different purpose, regardless of the flags.
> +	 */
> +	} else if (flag != st->current_flags || level != st->level ||
> +		   addr >= st->marker[1].start_address) {
> +
> +		/* Check the PTE flags */
> +		if (st->current_flags) {
> +			dump_addr(st, addr);
> +
> +			/* Dump all the flags */
> +			if (pg_level[st->level].flag)
> +				dump_flag(st, pg_level[st->level].flag,
> +					  pg_level[st->level].num);
> +			seq_puts(st->seq, "\n");
> +		}
> +
> +		/* Address indicates we have passed the end of the
> +		 * current section of virtual memory
> +		 */
> +		while (addr >= st->marker[1].start_address) {
> +			st->marker++;
> +			seq_printf(st->seq, "---[ %s ]---\n", st->marker->name);
> +		}
> +		st->start_address = addr;
> +		st->current_flags = flag;
> +		st->level = level;
> +	}
> +}
> +
> +static void walk_pte(struct pg_state *st, pmd_t *pmd, unsigned long start)
> +{
> +	pte_t *pte = pte_offset_kernel(pmd, 0);
> +	unsigned long addr;
> +	unsigned i;
> +
> +	for (i = 0; i < PTRS_PER_PTE; i++, pte++) {
> +		addr = start + i * PAGE_SIZE;
> +		note_page(st, addr, 4, pte_val(*pte));
> +	}
> +}
> +
> +static void walk_pmd(struct pg_state *st, pud_t *pud, unsigned long start)
> +{
> +	pmd_t *pmd = pmd_offset(pud, 0);
> +	unsigned long addr;
> +	unsigned i;
> +
> +	for (i = 0; i < PTRS_PER_PMD; i++, pmd++) {
> +		addr = start + i * PMD_SIZE;
> +		if (!pmd_none(*pmd))
> +			/* pmd exists */
> +			walk_pte(st, pmd, addr);
> +		else
> +			note_page(st, addr, 3, pmd_val(*pmd));
> +	}
> +}
> +
> +static void walk_pud(struct pg_state *st, pgd_t *pgd, unsigned long start)
> +{
> +	pud_t *pud = pud_offset(pgd, 0);
> +	unsigned long addr;
> +	unsigned i;
> +
> +	for (i = 0; i < PTRS_PER_PUD; i++, pud++) {
> +		addr = start + i * PUD_SIZE;
> +		if (!pud_none(*pud))
> +			/* pud exists */
> +			walk_pmd(st, pud, addr);
> +		else
> +			note_page(st, addr, 2, pud_val(*pud));
> +	}
> +}
> +
> +static void walk_pgd(struct pg_state *st, unsigned long start)
> +{
> +	pgd_t *pgd = pgd_offset_k(0UL);
> +	unsigned i;
> +	unsigned long addr;
> +
> +	for (i = 0; i < PTRS_PER_PGD; i++, pgd++) {
> +		addr = start + i * PGDIR_SIZE;
> +		if (!pgd_none(*pgd))
> +			/* pgd exists */
> +			walk_pud(st, pgd, addr);
> +		else
> +			note_page(st, addr, 1, pgd_val(*pgd));
> +	}
> +}
> +
> +static int ptdump_show(struct seq_file *m, void *v)
> +{
> +	struct pg_state st = {
> +		.seq = m,
> +		.start_address = KERN_VIRT_START,
> +		.marker = address_markers,
> +	};
> +	/* Traverse kernel page tables */
> +	walk_pgd(&st, KERN_VIRT_START);
> +	note_page(&st, 0, 0, 0);
> +	return 0;
> +}
> +
> +static int ptdump_open(struct inode *inode, struct file *file)
> +{
> +	return single_open(file, ptdump_show, NULL);
> +}
> +
> +static const struct file_operations ptdump_fops = {
> +	.open		= ptdump_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
> +
> +static void build_pgtable_complete_mask(void)
> +{
> +	unsigned i, j;
> +
> +	for (i = 0; i < ARRAY_SIZE(pg_level); i++)
> +		if (pg_level[i].flag)
> +			for (j = 0; j < pg_level[i].num; j++)
> +				pg_level[i].mask |= pg_level[i].flag[j].mask;
> +}
> +
> +static int ptdump_init(void)
> +{
> +	struct dentry *debugfs_file;
> +
> +	build_pgtable_complete_mask();
> +	debugfs_file = debugfs_create_file("kernel_pagetables", 0400, NULL,
> +			NULL, &ptdump_fops);
> +	return debugfs_file ? 0 : -ENOMEM;
> +}
> +device_initcall(ptdump_init);