[PATCH 12/12] x86/mm: Simplify pgd_alloc()

[Ingo Molnar <mingo@kernel.org>]

Right now pgd_alloc() uses pgd_ctor(), which copies over the
current swapper_pg_dir[] to a new task's PGD.

This is not necessary, it's enough if we clear it: the PGD will
then be properly updated by arch_pgd_init_late().

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/mm/pgtable.c | 27 +++++++++------------------
 1 file changed, 9 insertions(+), 18 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 7cca42c01677..d7d341e57e33 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -87,20 +87,6 @@ void ___pud_free_tlb(struct mmu_gather *tlb, pud_t *pud)
 #define UNSHARED_PTRS_PER_PGD				\
 	(SHARED_KERNEL_PMD ? KERNEL_PGD_BOUNDARY : PTRS_PER_PGD)
 
-static void pgd_ctor(struct mm_struct *mm, pgd_t *pgd)
-{
-	/* 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) {
-		clone_pgd_range(pgd + KERNEL_PGD_BOUNDARY,
-				swapper_pg_dir + KERNEL_PGD_BOUNDARY,
-				KERNEL_PGD_PTRS);
-	}
-}
-
 /*
  * List of all pgd's needed for non-PAE so it can invalidate entries
  * in both cached and uncached pgd's; not needed for PAE since the
@@ -328,11 +314,16 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 		goto out_free_pmds;
 
 	/*
-	 * No locking is needed here, as the PGD is still private,
-	 * so no code walking the task list and looking at mm->pgd
-	 * will be able to see it before it's fully constructed:
+	 * Zero out the kernel portion here, we'll set them up in
+	 * arch_pgd_init_late(), when the pgd is globally
+	 * visible already per the task list, so that it cannot
+	 * miss updates.
+	 *
+	 * We need to zero it here, to make sure arch_pgd_init_late()
+	 * can initialize them without locking.
 	 */
-	pgd_ctor(mm, pgd);
+	memset(pgd + KERNEL_PGD_BOUNDARY, 0, KERNEL_PGD_PTRS*sizeof(pgd_t));
+
 	pgd_prepopulate_pmd(mm, pgd, pmds);
 
 	return pgd;
-- 
2.1.4