Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Michal Hocko]

On Wed 31-05-17 08:30:08, Vlastimil Babka wrote:
> On 05/30/2017 06:06 PM, Andrea Arcangeli wrote:
> > 
> > I'm not sure if it should be considered a bug, the prctl is intended
> > to use normally by wrappers so it looks optimal as implemented this
> > way: affecting future vmas only, which will all be created after
> > execve executed by the wrapper.
> > 
> > What's the point of messing with the prctl so it mangles over the
> > wrapper process own vmas before exec? Messing with those vmas is pure
> > wasted CPUs for the wrapper use case which is what the prctl was
> > created for.
> > 
> > Furthermore there would be the risk a program that uses the prctl not
> > as a wrapper and then calls the prctl to clear VM_NOHUGEPAGE from
> > def_flags assuming the current kABI. The program could assume those
> > vmas that were instantiated before disabling the prctl are still with
> > VM_NOHUGEPAGE set (they would not after the change you propose).
> > 
> > Adding a scan of all vmas to PR_SET_THP_DISABLE to clear VM_NOHUGEPAGE
> > on existing vmas looks more complex too and less finegrined so
> > probably more complex for userland to manage
> 
> I would expect the prctl wouldn't iterate all vma's, nor would it modify
> def_flags anymore. It would just set a flag somewhere in mm struct that
> would be considered in addition to the per-vma flags when deciding
> whether to use THP.

Exactly. Something like the below (not even compile tested).

> We could consider whether MADV_HUGEPAGE should be
> able to override the prctl or not.

This should be a master override to any per vma setting.

---
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index a3762d49ba39..9da053ced864 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -92,6 +92,7 @@ extern bool is_vma_temporary_stack(struct vm_area_struct *vma);
 	   (1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG) &&			\
 	   ((__vma)->vm_flags & VM_HUGEPAGE))) &&			\
 	 !((__vma)->vm_flags & VM_NOHUGEPAGE) &&			\
+	 !test_bit(MMF_DISABLE_THP, &(__vma)->vm_mm->flags) &&		\
 	 !is_vma_temporary_stack(__vma))
 #define transparent_hugepage_use_zero_page()				\
 	(transparent_hugepage_flags &					\
diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 5d9a400af509..f0d7335336cd 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -48,7 +48,8 @@ static inline int khugepaged_enter(struct vm_area_struct *vma,
 	if (!test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
 		if ((khugepaged_always() ||
 		     (khugepaged_req_madv() && (vm_flags & VM_HUGEPAGE))) &&
-		    !(vm_flags & VM_NOHUGEPAGE))
+		    !(vm_flags & VM_NOHUGEPAGE) &&
+		    !test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
 			if (__khugepaged_enter(vma->vm_mm))
 				return -ENOMEM;
 	return 0;
diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 69eedcef8f03..2c07b244090a 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -68,6 +68,7 @@ static inline int get_dumpable(struct mm_struct *mm)
 #define MMF_OOM_SKIP		21	/* mm is of no interest for the OOM killer */
 #define MMF_UNSTABLE		22	/* mm is unstable for copy_from_user */
 #define MMF_HUGE_ZERO_PAGE	23      /* mm has ever used the global huge zero page */
+#define MMF_DISABLE_THP		24	/* disable THP for all VMAs */
 
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK)
 
diff --git a/kernel/sys.c b/kernel/sys.c
index 8a94b4eabcaa..e48f0636c7fd 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2266,7 +2266,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_GET_THP_DISABLE:
 		if (arg2 || arg3 || arg4 || arg5)
 			return -EINVAL;
-		error = !!(me->mm->def_flags & VM_NOHUGEPAGE);
+		error = !!test_bit(MMF_DISABLE_THP, &me->mm->flags);
 		break;
 	case PR_SET_THP_DISABLE:
 		if (arg3 || arg4 || arg5)
@@ -2274,9 +2274,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		if (down_write_killable(&me->mm->mmap_sem))
 			return -EINTR;
 		if (arg2)
-			me->mm->def_flags |= VM_NOHUGEPAGE;
+			set_bit(MMF_DISABLE_THP, &me->mm->flags);
 		else
-			me->mm->def_flags &= ~VM_NOHUGEPAGE;
+			clear_bit(MMF_DISABLE_THP, &me->mm->flags);
 		up_write(&me->mm->mmap_sem);
 		break;
 	case PR_MPX_ENABLE_MANAGEMENT:
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index ce29e5cc7809..57e31f4752b3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -818,7 +818,8 @@ khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
 static bool hugepage_vma_check(struct vm_area_struct *vma)
 {
 	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
-	    (vma->vm_flags & VM_NOHUGEPAGE))
+	    (vma->vm_flags & VM_NOHUGEPAGE) ||
+	    test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
 		return false;
 	if (shmem_file(vma->vm_file)) {
 		if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE))
diff --git a/mm/shmem.c b/mm/shmem.c
index e67d6ba4e98e..27fe1bbf813b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1977,10 +1977,11 @@ static int shmem_fault(struct vm_fault *vmf)
 	}
 
 	sgp = SGP_CACHE;
-	if (vma->vm_flags & VM_HUGEPAGE)
-		sgp = SGP_HUGE;
-	else if (vma->vm_flags & VM_NOHUGEPAGE)
+	
+	if ((vma->vm_flags & VM_NOHUGEPAGE) || test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
 		sgp = SGP_NOHUGE;
+	else if (vma->vm_flags & VM_HUGEPAGE)
+		sgp = SGP_HUGE;
 
 	error = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp,
 				  gfp, vma, vmf, &ret);
-- 
Michal Hocko
SUSE Labs