linux-kernel - [PATCH 2/4] mm: mlock: avoid increase mm->locked_vm on mlock() when already mlock2(,MLOCK

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Message-Id: <1472554781-9835-3-git-send-email-wei.guo.simon@gmail.com>
Date:   Tue, 30 Aug 2016 18:59:39 +0800
From:   wei.guo.simon@...il.com
To:     linux-mm@...ck.org
Cc:     Alexey Klimov <klimov.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Eric B Munson <emunson@...mai.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>, Shuah Khan <shuah@...nel.org>,
        Simon Guo <wei.guo.simon@...il.com>,
        Thierry Reding <treding@...dia.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH 2/4] mm: mlock: avoid increase mm->locked_vm on mlock() when already mlock2(,MLOCK_ONFAULT)

From: Simon Guo <wei.guo.simon@...il.com>

When one vma was with flag VM_LOCKED|VM_LOCKONFAULT (by invoking
mlock2(,MLOCK_ONFAULT)), it can again be populated with mlock() with
VM_LOCKED flag only.

There is a hole in mlock_fixup() which increase mm->locked_vm twice even
the two operations are on the same vma and both with VM_LOCKED flags.

The issue can be reproduced by following code:
mlock2(p, 1024 * 64, MLOCK_ONFAULT); //VM_LOCKED|VM_LOCKONFAULT
mlock(p, 1024 * 64);  //VM_LOCKED
Then check the increase VmLck field in /proc/pid/status(to 128k).

When vma is set with different vm_flags, and the new vm_flags is with
VM_LOCKED, it is not necessarily be a "new locked" vma.  This patch
corrects this bug by prevent mm->locked_vm from increment when old
vm_flags is already VM_LOCKED.

Signed-off-by: Simon Guo <wei.guo.simon@...il.com>
---
 mm/mlock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index 9283187..df29aad 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -516,6 +516,7 @@ static int mlock_fixup(struct vm_area_struct *vma, struct vm_area_struct **prev,
 	int nr_pages;
 	int ret = 0;
 	int lock = !!(newflags & VM_LOCKED);
+	vm_flags_t old_flags = vma->vm_flags;

 	if (newflags == vma->vm_flags || (vma->vm_flags & VM_SPECIAL) ||
 	    is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm))
@@ -550,6 +551,8 @@ success:
 	nr_pages = (end - start) >> PAGE_SHIFT;
 	if (!lock)
 		nr_pages = -nr_pages;
+	else if (old_flags & VM_LOCKED)
+		nr_pages = 0;
 	mm->locked_vm += nr_pages;

 	/*
-- 
1.8.3.1