linux-kernel - Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

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Date:   Thu, 15 Mar 2018 14:51:06 +0000
From:   "Kani, Toshi" <toshi.kani@....com>
To:     "bp@...e.de" <bp@...e.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "Hocko, Michal" <mhocko@...e.com>,
        "cpandya@...eaurora.org" <cpandya@...eaurora.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "guohanjun@...wei.com" <guohanjun@...wei.com>,
        "wxf.wang@...ilicon.com" <wxf.wang@...ilicon.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/2] x86/mm: implement free pmd/pte page interfaces

On Thu, 2018-03-15 at 13:09 +0530, Chintan Pandya wrote:
> 
> On 3/14/2018 11:31 PM, Toshi Kani wrote:
> > Implement pud_free_pmd_page() and pmd_free_pte_page() on x86, which
> > clear a given pud/pmd entry and free up lower level page table(s).
> > Address range associated with the pud/pmd entry must have been purged
> > by INVLPG.
> > 
> > fixes: e61ce6ade404e ("mm: change ioremap to set up huge I/O mappings")
> > Signed-off-by: Toshi Kani <toshi.kani@....com>
> > Cc: Michal Hocko <mhocko@...e.com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: Matthew Wilcox <willy@...radead.org>
> > Cc: <stable@...r.kernel.org>
> > ---
> >   arch/x86/mm/pgtable.c |   28 ++++++++++++++++++++++++++--
> >   1 file changed, 26 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index 1eed7ed518e6..34cda7e0551b 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -712,7 +712,22 @@ int pmd_clear_huge(pmd_t *pmd)
> >    */
> >   int pud_free_pmd_page(pud_t *pud)
> >   {
> > -	return pud_none(*pud);
> > +	pmd_t *pmd;
> > +	int i;
> > +
> > +	if (pud_none(*pud))
> > +		return 1;
> > +
> > +	pmd = (pmd_t *)pud_page_vaddr(*pud);
> > +
> > +	for (i = 0; i < PTRS_PER_PMD; i++)
> > +		if (!pmd_free_pte_page(&pmd[i]))
> 
> This is forced action and no optional. Also, pmd_free_pte_page()
> doesn't return 0 in any case. So, you may remove _if_ ?

The code needs to be written per the interface definition, not per the
current implementation.

> > +			return 0;
> > +
> > +	pud_clear(pud);
> > +	free_page((unsigned long)pmd);
> > +
> > +	return 1;
> >   }
> >   
> >   /**
> > @@ -724,6 +739,15 @@ int pud_free_pmd_page(pud_t *pud)
> >    */
> >   int pmd_free_pte_page(pmd_t *pmd)
> >   {
> > -	return pmd_none(*pmd);
> > +	pte_t *pte;
> > +
> > +	if (pmd_none(*pmd))
> 
> This should also check if pmd is already huge. Same for pud ?

Not necessary. As described in the function header, one of the entry
conditions is that a given pmd range is unmapped.  See
vunmap_pmd_range().

Thanks,
-Toshi