linux-kernel - Re: [PATCH v12 4/5] arm64: Implement page table free interfaces

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Message-ID: <98ef3cd0-a9a1-d5b5-f1a6-c0ab8b15ec6a@codeaurora.org>
Date:   Mon, 4 Jun 2018 19:13:18 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Will Deacon <will.deacon@....com>
Cc:     catalin.marinas@....com, mark.rutland@....com,
        akpm@...ux-foundation.org, toshi.kani@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 4/5] arm64: Implement page table free interfaces



On 6/4/2018 5:43 PM, Will Deacon wrote:
> On Fri, Jun 01, 2018 at 06:09:17PM +0530, Chintan Pandya wrote:
>> Implement pud_free_pmd_page() and pmd_free_pte_page().
>>
>> Implementation requires,
>>   1) Clearing off the current pud/pmd entry
>>   2) Invalidate TLB which could have previously
>>      valid but not stale entry
>>   3) Freeing of the un-used next level page tables
> 
> Please can you rewrite this describing the problem that you're solving,
> rather than a brief summary of some requirements?

Okay. I'll fix this in v13.

> 
>> Signed-off-by: Chintan Pandya <cpandya@...eaurora.org>
>> ---
>>   arch/arm64/mm/mmu.c | 38 ++++++++++++++++++++++++++++++++++----
>>   1 file changed, 34 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 8ae5d7a..6e7e16c 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -45,6 +45,7 @@
>>   #include <asm/memblock.h>
>>   #include <asm/mmu_context.h>
>>   #include <asm/ptdump.h>
>> +#include <asm/tlbflush.h>
>>   
>>   #define NO_BLOCK_MAPPINGS	BIT(0)
>>   #define NO_CONT_MAPPINGS	BIT(1)
>> @@ -977,12 +978,41 @@ int pmd_clear_huge(pmd_t *pmdp)
>>   	return 1;
>>   }
>>   
>> -int pud_free_pmd_page(pud_t *pud, unsigned long addr)
>> +int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>>   {
>> -	return pud_none(*pud);
>> +	pte_t *table;
>> +	pmd_t pmd;
>> +
>> +	pmd = READ_ONCE(*pmdp);
>> +	if (pmd_present(pmd)) {
>> +		table = pmd_page_vaddr(pmd);
>> +		pmd_clear(pmdp);
>> +		__flush_tlb_kernel_pgtable(addr);
>> +		pte_free_kernel(NULL, table);
>> +	}
>> +	return 1;
>>   }
>>   
>> -int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
>> +int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>>   {
>> -	return pmd_none(*pmd);
>> +	pmd_t *table;
>> +	pmd_t *entry;
>> +	pud_t pud;
>> +	unsigned long next, end;
>> +
>> +	pud = READ_ONCE(*pudp);
>> +	if (pud_present(pud)) {
> 
> Just some stylistic stuff, but please can you rewrite this as:
> 
> 	if (!pud_present(pud) || VM_WARN_ON(!pud_table(pud)))
> 		return 1;
> 
> similarly for the pmd/pte code above.

Okay. v13 will have this.

> 
>> +		table = pud_page_vaddr(pud);
>> +		entry = table;
> 
> Could you rename entry -> pmdp, please?

Sure.

> 
>> +		next = addr;
>> +		end = addr + PUD_SIZE;
>> +		do {
>> +			pmd_free_pte_page(entry, next);
>> +		} while (entry++, next += PMD_SIZE, next != end);
>> +
>> +		pud_clear(pudp);
>> +		__flush_tlb_kernel_pgtable(addr);
>> +		pmd_free(NULL, table);
>> +	}
>> +	return 1;
> 
> So with these patches, we only ever return 1 from these helpers. It looks
> like the same is true for x86, so how about we make them void and move the
> calls inside the conditionals in lib/ioremap.c? Obviously, this would be a
> separate patch on the end.

That sounds valid code churn to me. But since x86 discussion is not
concluded yet, I would wait to share until that gets resolved. May be
not in v13 but separate effort. Would that be okay to you ?

> 
> Will
> 

Chintan
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