Hi,
On Mon, Jan 25, 2016 at 08:52:13AM -0800, Laura Abbott wrote:
> Under some circumstances (e.g. debugging) it may be useful to have all
> kernel memory mapped using PAGE_SIZE pages. Add an option for this.
>
> Signed-off-by: Laura Abbott <[email protected]>
> ---
> arch/arm64/Kconfig | 11 +++++++++++
> arch/arm64/mm/mmu.c | 10 +++++++---
> 2 files changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index ffa3c54..faf7eac 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -504,6 +504,17 @@ config HOTPLUG_CPU
> Say Y here to experiment with turning CPUs off and on. CPUs
> can be controlled through /sys/devices/system/cpu.
>
> +config FORCE_PAGES
> + bool "Force all memory mappings to be PAGE_SIZE"
> + help
> + For performance reasons, kernel memory may be mapped with
> + page table entries larger than the expected PAGE_SIZE. This results
> + in better TLB performance but prevents adjustment of page table
> + attributes at runtime. Say Y here to have all kernel memory mapped
> + with PAGE_SIZE entries.
> +
> + If unsure, say N.
> +
I think we can make this a boot-time option, given that we're simply
flipping a boolean that only directly affects a small amount of boot
code. I doubt there would be a noticeable overhead, and it makes it more
likely that we can use this to debug production kernels.
Is there a usecase other than DEBUG_PAGEALLOC that you have in mind? If
not, we could just hang this off of debug_pagealloc_enabled(), as we
parse the early params before we create the usual page tables.
Thanks,
Mark.
> source kernel/Kconfig.preempt
> source kernel/Kconfig.hz
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 2d6e7cf..450d38a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -181,7 +181,8 @@ static void alloc_init_pmd(pud_t *pud, unsigned long
> addr, unsigned long end,
> do {
> next = pmd_addr_end(addr, end);
> /* try section mapping first */
> - if (((addr | next | phys) & ~SECTION_MASK) == 0) {
> + if (((addr | next | phys) & ~SECTION_MASK) == 0 &&
> + (!IS_ENABLED(CONFIG_FORCE_PAGES) || !pgtable_alloc)) {
> pmd_t old_pmd =*pmd;
> set_pmd(pmd, __pmd(phys |
> pgprot_val(mk_sect_prot(prot))));
> @@ -208,8 +209,11 @@ static void alloc_init_pmd(pud_t *pud, unsigned long
> addr, unsigned long end,
> }
>
> static inline bool use_1G_block(unsigned long addr, unsigned long next,
> - unsigned long phys)
> + unsigned long phys, phys_addr_t (*pgtable_alloc)(void))
> {
> + if (pgtable_alloc && IS_ENABLED(CONFIG_FORCE_PAGES))
> + return false;
> +
> if (PAGE_SHIFT != 12)
> return false;
>
> @@ -241,7 +245,7 @@ static void alloc_init_pud(pgd_t *pgd, unsigned long
> addr, unsigned long end,
> /*
> * For 4K granule only, attempt to put down a 1GB block
> */
> - if (use_1G_block(addr, next, phys)) {
> + if (use_1G_block(addr, next, phys, pgtable_alloc)) {
> pud_t old_pud = *pud;
> set_pud(pud, __pud(phys |
> pgprot_val(mk_sect_prot(prot))));
> --
> 2.5.0
>
