From c1294d05de5df1ab8c93aa13c531782ede907e14 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <aarcange@redhat.com>
Date: Fri, 4 Sep 2015 15:46:27 -0700
Subject: [PATCH] userfaultfd: prevent khugepaged to merge if userfaultfd is
 armed

If userfaultfd is armed on a certain vma we can't "fill" the holes with
zeroes or we'll break the userland on demand paging.  The holes if the
userfault is armed, are really missing information (not zeroes) that the
userland has to load from network or elsewhere.

The same issue happens for wrprotected ptes that we can't just convert
into a single writable pmd_trans_huge.

We could however in theory still merge across zeropages if only
VM_UFFD_MISSING is set (so if VM_UFFD_WP is not set)...  that could be
slightly improved but it'd be much more complex code for a tiny corner
case.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Cc: Sanidhya Kashyap <sanidhya.gatech@gmail.com>
Cc: zhang.zhanghailiang@huawei.com
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andres Lagar-Cavilla <andreslc@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Feiner <pfeiner@google.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 mm/huge_memory.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 7735f99931fa..d38aaf9dcba6 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2158,7 +2158,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
 	     _pte++, address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			if (++none_or_zero <= khugepaged_max_ptes_none)
+			if (!userfaultfd_armed(vma) &&
+			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else
 				goto out;
@@ -2611,7 +2612,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
 	     _pte++, _address += PAGE_SIZE) {
 		pte_t pteval = *_pte;
 		if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
-			if (++none_or_zero <= khugepaged_max_ptes_none)
+			if (!userfaultfd_armed(vma) &&
+			    ++none_or_zero <= khugepaged_max_ptes_none)
 				continue;
 			else
 				goto out_unmap;