linux-kernel - Re: [PATCH v5 3/4] arm64: Implement page table free interfaces

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Date:   Wed, 28 Mar 2018 12:29:25 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Will Deacon <will.deacon@....com>
Cc:     catalin.marinas@....com, mark.rutland@....com, toshi.kani@....com,
        arnd@...db.de, ard.biesheuvel@...aro.org, marc.zyngier@....com,
        james.morse@....com, kristina.martsenko@....com,
        takahiro.akashi@...aro.org, gregkh@...uxfoundation.org,
        tglx@...utronix.de, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH v5 3/4] arm64: Implement page table free interfaces



On 3/27/2018 11:30 PM, Will Deacon wrote:
> Hi Chintan,
Hi Will,

> 
> On Tue, Mar 27, 2018 at 06:54:59PM +0530, Chintan Pandya wrote:
>> Implement pud_free_pmd_page() and pmd_free_pte_page().
>>
>> Implementation requires,
>>   1) Freeing of the un-used next level page tables
>>   2) Clearing off the current pud/pmd entry
>>   3) Invalidate TLB which could have previously
>>      valid but not stale entry
>>
>> Signed-off-by: Chintan Pandya <cpandya@...eaurora.org>
>> ---
>> V4->V5:
>>   - Using __flush_tlb_kernel_pgtable instead of
>>     flush_tlb_kernel_range
>>
>>
>>   arch/arm64/mm/mmu.c | 33 +++++++++++++++++++++++++++++++--
>>   1 file changed, 31 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index da98828..3552c7a 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)
>> @@ -973,12 +974,40 @@ int pmd_clear_huge(pmd_t *pmdp)
>>   	return 1;
>>   }
>>   
>> +static int __pmd_free_pte_page(pmd_t *pmd, unsigned long addr, bool tlb_inv)
>> +{
>> +	pmd_t *table;
>> +
>> +	if (pmd_val(*pmd)) {
> 
> Please can you follow what I did in 20a004e7b017 ("arm64: mm: Use
> READ_ONCE/WRITE_ONCE when accessing page tables") and:
> 
>    1. Use consistent naming, so pmd_t * pmdp.
>    2. Use READ_ONCE to dereference the entry once into a local.
> 
> Similarly for the pud code below.

Sure. I'll fix this in v6.

> 
>> +		table = __va(pmd_val(*pmd));
>> +		pmd_clear(pmd);
>> +		if (tlb_inv)
>> +			__flush_tlb_kernel_pgtable(addr);
>> +
>> +		free_page((unsigned long) table);
> 
> Hmm. Surely it's only safe to call free_page if !tlb_inv in situations when
> the page table is already disconnected at a higher level? That doesn't
> appear to be the case with the function below, which still has the pud
> installed. What am I missing?
> 

Point ! Without the invalidation, free'ing a page is not safe. Better, I
do __flush_tlb_kernel_pgtable() every time. This might not be as costly
as flush_tlb_kernel_range().

>> +	}
>> +	return 1;
>> +}
>> +
>>   int pud_free_pmd_page(pud_t *pud, unsigned long addr)
>>   {
>> -	return pud_none(*pud);
>> +	pmd_t *table;
>> +	int i;
>> +
>> +	if (pud_val(*pud)) {
>> +		table = __va(pud_val(*pud));
>> +		for (i = 0; i < PTRS_PER_PMD; i++)
>> +			__pmd_free_pte_page(&table[i], addr + (i * PMD_SIZE),
>> +						false);
>> +
>> +		pud_clear(pud);
>> +		flush_tlb_kernel_range(addr, addr + PUD_SIZE);
> 
> Why aren't you using __flush_tlb_kernel_pgtable here?
> 

Now that I will call __flush_tlb_kernel_pgtable() for every  PMD, I can
use __flush_tlb_kernel_pgtable() here as well.

Previously, the thought was, while invalidating PUD by VA would not work
always because PUD may have next level of valid mapping still present in
the table (valid next PMD but invalid next-to-next PTE). In this case
doing just __flush_tlb_kernel_pgtable() for PUD might not be enough. We
need to invalidate subsequent tables as well which I was skipping for 
optimization. So, I used flush_tlb_kernel_range().

I will upload v6.

> Will
> 

Chintan
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