Re: [PATCH v3 2/3] arm64: Implement page table free interfaces

[Chintan Pandya]

On 3/20/2018 12:59 AM, Kani, Toshi wrote:
> On Mon, 2018-03-19 at 18:10 +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@xxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/mm/mmu.c | 30 ++++++++++++++++++++++++++++--
> >   1 file changed, 28 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index da98828..c70f139 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)
> > @@ -975,10 +976,35 @@ int pmd_clear_huge(pmd_t *pmdp)
> >   
> >   int pud_free_pmd_page(pud_t *pud, unsigned long addr)
> >   {
> > -	return pud_none(*pud);
> > +	pmd_t *pmd;
> > +	int i;
> > +
> > +	pmd = __va(pud_val(*pud));
> > +	if (pud_val(*pud)) {
> > +		for (i = 0; i < PTRS_PER_PMD; i++)
> > +			pmd_free_pte_page(&pmd[i], addr + (i * PMD_SIZE));
> > +
> > +		free_page((unsigned long) pmd);
>
> Why do you want to free this pmd page before clearing the pud entry on
> this arm64 version (as it seems you intentionally changed it from the
> x86 version)?  It can be reused while being pointed by the pud.  Same
> for pmd.
Noted.
> > +		pud_clear(pud);
> > +		flush_tlb_kernel_range(addr, addr + PUD_SIZE);
>
> Since you purge the entire pud range here, do you still need to call
> pmd_free_pte_page() to purge each pmd range?  This looks very expensive.
> You may want to consider if calling internal __pmd_free_pte_page()
> without the purge operation works.
I completely missed that. Sure, will fix this.

I will upload v4 fixing all 4 comments.
> -Toshi

Chintan
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