linux-kernel - Re: [PATCH v2 2/4] ioremap: Implement TLB

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Message-ID: <20180315152045.uajoedfvdcynhus5@lakrids.cambridge.arm.com>
Date:   Thu, 15 Mar 2018 15:20:45 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Chintan Pandya <cpandya@...eaurora.org>
Cc:     linux-arch@...r.kernel.org, toshi.kani@....com, arnd@...db.de,
        ard.biesheuvel@...aro.org, marc.zyngier@....com,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, kristina.martsenko@....com,
        takahiro.akashi@...aro.org, james.morse@....com,
        gregkh@...uxfoundation.org, tglx@...utronix.de,
        akpm@...ux-foundation.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/4] ioremap: Implement TLB_INV before huge mapping

On Thu, Mar 15, 2018 at 07:49:01PM +0530, Chintan Pandya wrote:
> On 3/15/2018 7:01 PM, Mark Rutland wrote:
> > On Thu, Mar 15, 2018 at 06:15:04PM +0530, Chintan Pandya wrote:
> > > @@ -91,10 +93,15 @@ static inline int ioremap_pmd_range(pud_t *pud, unsigned long addr,
> > >   		if (ioremap_pmd_enabled() &&
> > >   		    ((next - addr) == PMD_SIZE) &&
> > > -		    IS_ALIGNED(phys_addr + addr, PMD_SIZE) &&
> > > -		    pmd_free_pte_page(pmd)) {
> > > -			if (pmd_set_huge(pmd, phys_addr + addr, prot))
> > > +		    IS_ALIGNED(phys_addr + addr, PMD_SIZE)) {
> > > +			old_pmd = *pmd;
> > > +			pmd_clear(pmd);
> > > +			flush_tlb_pgtable(&init_mm, addr);
> > > +			if (pmd_set_huge(pmd, phys_addr + addr, prot)) {
> > > +				pmd_free_pte_page(&old_pmd);
> > >   				continue;
> > > +			} else
> > > +				set_pmd(pmd, old_pmd);
> > >   		}
> > 
> > Can we have something like a pmd_can_set_huge() helper? Then we could
> > avoid pointless modification and TLB invalidation work when
> > pmd_set_huge() will fail.
> 
> Actually, pmd_set_huge() will never fail because, if
> CONFIG_HAVE_ARCH_HUGE_VMAP is disabled, ioremap_pmd_enabled()
> will fail and if enabled (i.e. ARM64 & x86), they don't fail
> in their implementation. So, rather we can do the following.

AFAICT, that's not true. The x86 pmd_set_huge() can fail under certain
conditions:

int pmd_set_huge(pmd_t *pmd, phys_addr_t addr, pgprot_t prot)
{
	u8 mtrr, uniform;

	mtrr = mtrr_type_lookup(addr, addr + PMD_SIZE, &uniform);
	if ((mtrr != MTRR_TYPE_INVALID) && (!uniform) &&
	    (mtrr != MTRR_TYPE_WRBACK)) {
		pr_warn_once("%s: Cannot satisfy [mem %#010llx-%#010llx] with a huge-page mapping due to MTRR override.\n",
			     __func__, addr, addr + PMD_SIZE);
		return 0;
	}

	prot = pgprot_4k_2_large(prot);

	set_pte((pte_t *)pmd, pfn_pte(
		(u64)addr >> PAGE_SHIFT,
		__pgprot(pgprot_val(prot) | _PAGE_PSE)));

	return 1;
}

... perhaps that can never happen in this particular case, but that's
not clear to me.

Thanks,
Mark.