Re: [RFC][PATCH 2/4] arm64: Add option to force mapping with PAGE_SIZE pages

[Laura Abbott]

On 01/26/2016 03:14 AM, Mark Rutland wrote:
> 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 <labbott@xxxxxxxxxxxxxxxxx>
> > ---
> >   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.

I only had DEBUG_PAGEALLOC use case in mind but I'm not sure about linking
the two completely. I'll see about adding a boot option instead of the
Kconfig option and then disabling debug_pagealloc if it isn't set.

Thanks,
Laura
 
> 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