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Date:	Mon, 6 Oct 2014 18:03:51 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Suleiman Souhlal <suleiman@...gle.com>, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch for-3.17] mm, thp: fix collapsing of hugepages on madvise

On Sat, Oct 04, 2014 at 07:48:04PM -0700, David Rientjes wrote:
> If an anonymous mapping is not allowed to fault thp memory and then
> madvise(MADV_HUGEPAGE) is used after fault, khugepaged will never
> collapse this memory into thp memory.
> 
> This occurs because the madvise(2) handler for thp, hugepage_advise(),
> clears VM_NOHUGEPAGE on the stack and it isn't stored in vma->vm_flags
> until the final action of madvise_behavior().  This causes the
> khugepaged_enter_vma_merge() to be a no-op in hugepage_advise() when the
> vma had previously had VM_NOHUGEPAGE set.
> 
> Fix this by passing the correct vma flags to the khugepaged mm slot
> handler.  There's no chance khugepaged can run on this vma until after
> madvise_behavior() returns since we hold mm->mmap_sem.
> 
> It would be possible to clear VM_NOHUGEPAGE directly from vma->vm_flags
> in hugepage_advise(), but I didn't want to introduce special case
> behavior into madvise_behavior().  I think it's best to just let it
> always set vma->vm_flags itself.
> 
> Cc: <stable@...r.kernel.org>
> Reported-by: Suleiman Souhlal <suleiman@...gle.com>
> Signed-off-by: David Rientjes <rientjes@...gle.com>

Okay, I've looked once again and it seems your approach is better.
Although, I don't like that we need to pass down vma->vm_flags to every
khugepaged_enter() and khugepaged_enter_vma_merge().

My proposal is below. Build-tested only.

And I don't think this is subject for stable@: no crash or serious
misbehaviour. Registering to khugepaged is postponed until first page
fault. Not a big deal.

>From 6434d87a313317bcbc98313794840c366ba39ba1 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Date: Mon, 6 Oct 2014 17:52:17 +0300
Subject: [PATCH] thp: fix registering VMA into khugepaged on 
 madvise(MADV_HUGEPAGE)

hugepage_madvise() tries to register VMA into khugepaged with
khugepaged_enter_vma_merge() on madvise(MADV_HUGEPAGE). Unfortunately
it's effectevely nop, since khugepaged_enter_vma_merge() rely on
vma->vm_flags which has not yet updated by the time of
hugepage_madvise().

Let's create a variant of khugepaged_enter_vma_merge() which takes
external vm_flags to consider. At the moment there's only two users for
such function: hugepage_madvise() and vma_merge().

Reported-by: Suleiman Souhlal <suleiman@...gle.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 include/linux/khugepaged.h |  9 +++++++++
 mm/huge_memory.c           | 27 ++++++++++++++++++++-------
 mm/mmap.c                  |  6 +++---
 3 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/include/linux/khugepaged.h b/include/linux/khugepaged.h
index 6b394f0b5148..bb25e323ee2d 100644
--- a/include/linux/khugepaged.h
+++ b/include/linux/khugepaged.h
@@ -7,6 +7,8 @@
 extern int __khugepaged_enter(struct mm_struct *mm);
 extern void __khugepaged_exit(struct mm_struct *mm);
 extern int khugepaged_enter_vma_merge(struct vm_area_struct *vma);
+extern int __khugepaged_enter_vma_merge(struct vm_area_struct *vma,
+		vm_flags_t vm_flags);
 
 #define khugepaged_enabled()					       \
 	(transparent_hugepage_flags &				       \
@@ -62,6 +64,13 @@ static inline int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
 {
 	return 0;
 }
+
+static inline int __khugepaged_enter_vma_merge(struct vm_area_struct *vma,
+		vm_flags_t vm_flags)
+{
+	return 0;
+}
+
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 #endif /* _LINUX_KHUGEPAGED_H */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 017aff657ef5..4b0afc076827 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1944,7 +1944,7 @@ out:
 #define VM_NO_THP (VM_SPECIAL | VM_HUGETLB | VM_SHARED | VM_MAYSHARE)
 
 int hugepage_madvise(struct vm_area_struct *vma,
-		     unsigned long *vm_flags, int advice)
+		     vm_flags_t *vm_flags, int advice)
 {
 	switch (advice) {
 	case MADV_HUGEPAGE:
@@ -1969,8 +1969,7 @@ int hugepage_madvise(struct vm_area_struct *vma,
 		 * register it here without waiting a page fault that
 		 * may not happen any time soon.
 		 */
-		if (unlikely(khugepaged_enter_vma_merge(vma)))
-			return -ENOMEM;
+		return __khugepaged_enter_vma_merge(vma, *vm_flags);
 		break;
 	case MADV_NOHUGEPAGE:
 		/*
@@ -2070,7 +2069,8 @@ int __khugepaged_enter(struct mm_struct *mm)
 	return 0;
 }
 
-int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
+int __khugepaged_enter_vma_merge(struct vm_area_struct *vma,
+		vm_flags_t vm_flags)
 {
 	unsigned long hstart, hend;
 	if (!vma->anon_vma)
@@ -2082,14 +2082,27 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
 	if (vma->vm_ops)
 		/* khugepaged not yet working on file or special mappings */
 		return 0;
-	VM_BUG_ON(vma->vm_flags & VM_NO_THP);
+	VM_BUG_ON(vm_flags & VM_NO_THP);
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
-	if (hstart < hend)
-		return khugepaged_enter(vma);
+	if (hstart >= hend)
+		return 0;
+	if (test_bit(MMF_VM_HUGEPAGE, &vma->vm_mm->flags))
+		return 0;
+	if (vm_flags & VM_NOHUGEPAGE)
+		return 0;
+	if (khugepaged_always())
+		return __khugepaged_enter(vma->vm_mm);
+	if (khugepaged_req_madv() && vm_flags & VM_HUGEPAGE)
+		return __khugepaged_enter(vma->vm_mm);
 	return 0;
 }
 
+int khugepaged_enter_vma_merge(struct vm_area_struct *vma)
+{
+	return __khugepaged_enter_vma_merge(vma, vma->vm_flags);
+}
+
 void __khugepaged_exit(struct mm_struct *mm)
 {
 	struct mm_slot *mm_slot;
diff --git a/mm/mmap.c b/mm/mmap.c
index c0a3637cdb64..19fee85f0b12 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1009,8 +1009,8 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
  */
 struct vm_area_struct *vma_merge(struct mm_struct *mm,
 			struct vm_area_struct *prev, unsigned long addr,
-			unsigned long end, unsigned long vm_flags,
-		     	struct anon_vma *anon_vma, struct file *file,
+			unsigned long end, vm_flags_t vm_flags,
+			struct anon_vma *anon_vma, struct file *file,
 			pgoff_t pgoff, struct mempolicy *policy)
 {
 	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
@@ -1056,7 +1056,7 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 				end, prev->vm_pgoff, NULL);
 		if (err)
 			return NULL;
-		khugepaged_enter_vma_merge(prev);
+		__khugepaged_enter_vma_merge(prev, vm_flags);
 		return prev;
 	}
 
-- 
 Kirill A. Shutemov
--
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