linux-kernel - Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init

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Date:	Thu, 11 Jun 2015 16:04:04 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
CC:	linux-mml@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [PATCH 06/12] x86/mm: Enable and use the arch_pgd_init_late()
 method

On 06/11/2015 10:07 AM, Ingo Molnar wrote:

> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index fb0a9dd1d6e4..e0bf90470d70 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -391,6 +391,63 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
>   	return NULL;
>   }
>
> +/*
> + * Initialize the kernel portion of the PGD.
> + *
> + * This is done separately, because pgd_alloc() happens when
> + * the task is not on the task list yet - and PGD updates
> + * happen by walking the task list.
> + *
> + * No locking is needed here, as we just copy over the reference
> + * PGD. The reference PGD (pgtable_init) is only ever expanded
> + * at the highest, PGD level. Thus any other task extending it
> + * will first update the reference PGD, then modify the task PGDs.
> + */
> +void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd)
> +{
> +	/*
> +	 * This is called after a new MM has been made visible
> +	 * in fork() or exec().
> +	 *
> +	 * This barrier makes sure the MM is visible to new RCU
> +	 * walkers before we initialize it, so that we don't miss
> +	 * updates:
> +	 */
> +	smp_wmb();
> +
> +	/*
> +	 * If the pgd points to a shared pagetable level (either the
> +	 * ptes in non-PAE, or shared PMD in PAE), then just copy the
> +	 * references from swapper_pg_dir:
> +	 */
> +	if (CONFIG_PGTABLE_LEVELS == 2 ||
> +	    (CONFIG_PGTABLE_LEVELS == 3 && SHARED_KERNEL_PMD) ||
> +	    CONFIG_PGTABLE_LEVELS == 4) {
> +
> +		pgd_t *pgd_src = swapper_pg_dir + KERNEL_PGD_BOUNDARY;
> +		pgd_t *pgd_dst =            pgd + KERNEL_PGD_BOUNDARY;
> +		int i;
> +
> +		for (i = 0; i < KERNEL_PGD_PTRS; i++, pgd_src++, pgd_dst++) {
> +			/*
> +			 * This is lock-less, so it can race with PGD updates
> +			 * coming from vmalloc() or CPA methods, but it's safe,
> +			 * because:
> +			 *
> +			 * 1) this PGD is not in use yet, we have still not
> +			 *    scheduled this task.
> +			 * 2) we only ever extend PGD entries
> +			 *
> +			 * So if we observe a non-zero PGD entry we can copy it,
> +			 * it won't change from under us. Parallel updates (new
> +			 * allocations) will modify our (already visible) PGD:
> +			 */
> +			if (pgd_val(*pgd_src))
> +				WRITE_ONCE(*pgd_dst, *pgd_src);


This should be set_pgd(pgd_dst, *pgd_src) in order for it to work as a 
Xen PV guest.

I don't know whether anything would need to be done wrt WRITE_ONCE. 
Perhaps put it into native_set_pgd()?

-boris


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