[PATCH 11/11] x86/mm: Simplify pgd_alloc()

Ingo Molnar
2015-09-22T06:27:31Z

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 cb5b8cbcf96b..ac72e60d5297 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