[PATCH 05/12] mm: Introduce arch_pgd_init_late()

[Ingo Molnar]

Add a late PGD init callback to places that allocate a new MM
with a new PGD: copy_process() and exec().

The purpose of this callback is to allow architectures to implement
lockless initialization of task PGDs, to remove the scalability
limit of pgd_list/pgd_lock.

Architectures can opt in to this callback via the ARCH_HAS_PGD_INIT_LATE
Kconfig flag. There's zero overhead on architectures that are not using it.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Waiman Long <Waiman.Long@xxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/Kconfig       |  9 +++++++++
 fs/exec.c          |  3 +++
 include/linux/mm.h |  6 ++++++
 kernel/fork.c      | 16 ++++++++++++++++
 4 files changed, 34 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index a65eafb24997..a8e866cd4247 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -491,6 +491,15 @@ config PGTABLE_LEVELS
 	int
 	default 2
 
+config ARCH_HAS_PGD_INIT_LATE
+	bool
+	help
+	  Architectures that want a late PGD initialization can define
+	  the arch_pgd_init_late() callback and it will be called
+	  by the generic new task (fork()) code after a new task has
+	  been made visible on the task list, but before it has been
+	  first scheduled.
+
 config ARCH_HAS_ELF_RANDOMIZE
 	bool
 	help
diff --git a/fs/exec.c b/fs/exec.c
index 1977c2a553ac..c1d213c64fda 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -860,7 +860,10 @@ static int exec_mmap(struct mm_struct *mm)
 	}
 	task_lock(tsk);
 	active_mm = tsk->active_mm;
+
 	tsk->mm = mm;
+	arch_pgd_init_late(mm, mm->pgd);
+
 	tsk->active_mm = mm;
 	activate_mm(active_mm, mm);
 	tsk->mm->vmacache_seqnum = 0;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0755b9fd03a7..35d887d2b038 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1134,6 +1134,12 @@ int follow_phys(struct vm_area_struct *vma, unsigned long address,
 int generic_access_phys(struct vm_area_struct *vma, unsigned long addr,
 			void *buf, int len, int write);
 
+#ifdef CONFIG_ARCH_HAS_PGD_INIT_LATE
+void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd);
+#else
+static inline void arch_pgd_init_late(struct mm_struct *mm, pgd_t *pgd) { }
+#endif
+
 static inline void unmap_shared_mapping_range(struct address_space *mapping,
 		loff_t const holebegin, loff_t const holelen)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index 03c1eaaa6ef5..1f83ceca6c6c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1592,6 +1592,22 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	syscall_tracepoint_update(p);
 	write_unlock_irq(&tasklist_lock);
 
+	/*
+	 * If we have a new PGD then initialize it:
+	 *
+	 * This method is called after a task has been made visible
+	 * on the task list already.
+	 *
+	 * Architectures that manage per task kernel pagetables
+	 * might use this callback to initialize them after they
+	 * are already visible to new updates.
+	 *
+	 * NOTE: any user-space parts of the PGD are already initialized
+	 *       and must not be clobbered.
+	 */
+	if (p->mm != current->mm)
+		arch_pgd_init_late(p->mm, p->mm->pgd);
+
 	proc_fork_connector(p);
 	cgroup_post_fork(p);
 	if (clone_flags & CLONE_THREAD)
-- 
2.1.4

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