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 <labb...@fedoraproject.org>
---
  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