Re: [PATCH v9 3/4] arm64: Implement page table free interfaces

[Will Deacon]

Hi Chintan,

[as a side note: I'm confused on the status of this patch series, as part
 of it was reposted separately by Toshi. Please can you work together?]

On Mon, Apr 30, 2018 at 01:11:33PM +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
> 
> Signed-off-by: Chintan Pandya <cpandya@xxxxxxxxxxxxxx>
> ---
>  arch/arm64/mm/mmu.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index da98828..0f651db 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 +974,32 @@ 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);
> +	pmd_t *table;
> +
> +	if (pmd_present(READ_ONCE(*pmdp))) {

Might also be worth checking pmd_table here, just in case. (same for pud)

> +		table = __va(pmd_val(*pmdp));

Can you avoid dereferencing *pmdp twice, and instead READ_ONCE into a local
variable, please? (same for pud)

> +		pmd_clear(pmdp);
> +		__flush_tlb_kernel_pgtable(addr);
> +		free_page((unsigned long) table);

Shouldn't this be pte_free_kernel, to pair with pte_alloc_kernel which
was used to allocate the page in the first place? (similarly for pud)

> +	}
> +	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;
> +	int i;
> +
> +	if (pud_present(READ_ONCE(*pudp))) {
> +		table = __va(pud_val(*pudp));
> +		for (i = 0; i < PTRS_PER_PMD; i++)
> +			pmd_free_pte_page(&table[i], addr + (i * PMD_SIZE));

I think it would be cleaner to write this as a do { ... } while, for
consistency with the ioremap and vmalloc code.

Will