[PATCH 0/2] mm->def_flags cleanups (Was: Change khugepaged to respect MMF_THP_DISABLE flag)

[Oleg Nesterov <oleg@redhat.com>]

Alex, Andrew, I think this simple series makes sense in any case,
but _perhaps_ it can also help THP_DISABLE.

On 01/20, Alex Thorlton wrote:
>
> On Mon, Jan 20, 2014 at 09:15:25PM +0100, Oleg Nesterov wrote:
> >
> > Although I got lost a bit, and probably misunderstood... but it
> > seems to me that whatever you do this patch should not touch
> > khugepaged_scan_mm_slot.
>
> Maybe I've gotten myself confused as well :)  After looking through the
> code some more, my understanding is that khugepaged_test_exit is used to
> make sure that __khugepaged_exit isn't running from underneath at certain
> times, so to have khugepaged_test_exit return true when __khugepaged_exit
> is not necessarily running, seems incorrect to me.

Still can't understand... probably I need to see v3.

But you know, I have another idea. Not sure you will like it, and probably
I missed something.

Can't we simply add VM_NOHUGEPAGE into ->def_flags? See the (untested)
patch below, on top of this series.

What do you think?

Oleg.


diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1cedd00..bc1dd9e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -167,6 +167,8 @@ extern unsigned int kobjsize(const void *objp);
  */
 #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP)
 
+#define VM_INIT_DEF_MASK	VM_NOHUGEPAGE
+
 /*
  * mapping from the currently active vm_flags protection bits (the
  * low four bits) to a page protection mask..
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 289760f..58afc04 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -149,4 +149,7 @@
 
 #define PR_GET_TID_ADDRESS	40
 
+#define PR_SET_THP_DISABLE	41
+#define PR_GET_THP_DISABLE	42
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/fork.c b/kernel/fork.c
index b84bef7..f6d020b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -529,8 +529,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
 	atomic_set(&mm->mm_count, 1);
 	init_rwsem(&mm->mmap_sem);
 	INIT_LIST_HEAD(&mm->mmlist);
-	mm->flags = (current->mm) ?
-		(current->mm->flags & MMF_INIT_MASK) : default_dump_filter;
 	mm->core_state = NULL;
 	atomic_long_set(&mm->nr_ptes, 0);
 	memset(&mm->rss_stat, 0, sizeof(mm->rss_stat));
@@ -538,8 +536,15 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p)
 	mm_init_aio(mm);
 	mm_init_owner(mm, p);
 
-	if (likely(!mm_alloc_pgd(mm))) {
+	if (current->mm) {
+		mm->flags = current->mm->flags & MMF_INIT_MASK;
+		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
+	} else {
+		mm->flags = default_dump_filter;
 		mm->def_flags = 0;
+	}
+
+	if (likely(!mm_alloc_pgd(mm))) {
 		mmu_notifier_mm_init(mm);
 		return mm;
 	}
diff --git a/kernel/sys.c b/kernel/sys.c
index ac1842e..eb8b0fc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2029,6 +2029,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		if (arg2 || arg3 || arg4 || arg5)
 			return -EINVAL;
 		return current->no_new_privs ? 1 : 0;
+	case PR_SET_THP_DISABLE:
+	case PR_GET_THP_DISABLE:
+		down_write(&me->mm->mmap_sem);
+		if (option == PR_SET_THP_DISABLE) {
+			if (arg2)
+				me->mm->def_flags |= VM_NOHUGEPAGE;
+			else
+				me->mm->def_flags &= ~VM_NOHUGEPAGE;
+		} else {
+			error = !!(me->mm->flags && VM_NOHUGEPAGE);
+		}
+		up_write(&me->mm->mmap_sem);
+		break;
 	default:
 		error = -EINVAL;
 		break;