linux-kernel - Re: [RFC][PATCH 2/4] arm64: Add option to force mapping with PAGE

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Message-ID: <56A7EA53.6080206@redhat.com>
Date:	Tue, 26 Jan 2016 13:51:15 -0800
From:	Laura Abbott <labbott@...hat.com>
To:	Mark Rutland <mark.rutland@....com>,
	Laura Abbott <labbott@...oraproject.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/4] arm64: Add option to force mapping with
 PAGE_SIZE pages

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@...oraproject.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
>>