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Date:   Fri, 24 Nov 2017 14:30:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 25/43] x86/mm/kaiser: Unmap kernel from userspace page
 tables (core patch)

On Fri, Nov 24, 2017 at 10:14:30AM +0100, Ingo Molnar wrote:
> +static pte_t *kaiser_shadow_pagetable_walk(unsigned long address,
> +					   unsigned long flags)
> +{
> +	pte_t *pte;
> +	pmd_t *pmd;
> +	pud_t *pud;
> +	p4d_t *p4d;
> +	pgd_t *pgd = kernel_to_shadow_pgdp(pgd_offset_k(address));
> +	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
> +
> +	if (flags & KAISER_WALK_ATOMIC) {
> +		gfp &= ~GFP_KERNEL;
> +		gfp |= __GFP_HIGH | __GFP_ATOMIC;
> +	}
> +
> +	if (address < PAGE_OFFSET) {
> +		WARN_ONCE(1, "attempt to walk user address\n");
> +		return NULL;
> +	}
> +
> +	if (pgd_none(*pgd)) {
> +		WARN_ONCE(1, "All shadow pgds should have been populated\n");
> +		return NULL;
> +	}
> +	BUILD_BUG_ON(pgd_large(*pgd) != 0);
> +
> +	p4d = p4d_offset(pgd, address);
> +	BUILD_BUG_ON(p4d_large(*p4d) != 0);
> +	if (p4d_none(*p4d)) {
> +		unsigned long new_pud_page = __get_free_page(gfp);
> +		if (!new_pud_page)
> +			return NULL;
> +
> +		spin_lock(&shadow_table_allocation_lock);
> +		if (p4d_none(*p4d))
> +			set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
> +		else
> +			free_page(new_pud_page);
> +		spin_unlock(&shadow_table_allocation_lock);

So mm/memory.c has two patterns here.. I prefer the other one:

		spin_lock(&shadow_table_allocation_lock);
		if (p4d_none(*p4d)) {
			set_p4d(p4d, __p4d(_KERNEL_TABLE | __pa(new_pud_page)));
			new_pud_page = NULL;
		}
		spin_unlock(&shadow_table_allocation_lock);
		if (new_pud_page)
			free_page(new_pud_page);

> +	}

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