Prepare for lockless PGD init: enable the arch_pgd_init_late() callback
and add a 'careful' implementation of PGD init to it: only copy over
non-zero entries.
Since PGD entries only ever get added, this method catches any updates
to swapper_pg_dir[] that might have occured between early PGD init
and late PGD init.
Note that this only matters for code that does not use the pgd_list but
the task list to find all PGDs in the system.
Subsequent patches will convert pgd_list users to task-list iterations.
[ This adds extra overhead in that we do the PGD initialization for a
second time - a later patch will simplify this, once we don't have
old pgd_list users. ]
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Andy Lutomirski <l...@amacapital.net>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Brian Gerst <brge...@gmail.com>
Cc: Denys Vlasenko <dvlas...@redhat.com>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
arch/x86/Kconfig | 1 +
arch/x86/mm/pgtable.c | 57
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 7e39f9b22705..15c19ce149f0 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -27,6 +27,7 @@ config X86
select ARCH_HAS_ELF_RANDOMIZE
select ARCH_HAS_FAST_MULTIPLIER
select ARCH_HAS_GCOV_PROFILE_ALL
+ select ARCH_HAS_PGD_INIT_LATE
select ARCH_HAS_SG_CHAIN
select ARCH_HAVE_NMI_SAFE_CMPXCHG
select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
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);
+ }
+ }
+}
+
void pgd_free(struct mm_struct *mm, pgd_t *pgd)
{
pgd_mop_up_pmds(mm, pgd);
--
2.1.4
--
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