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SubjectRe: [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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