[X][X/linux-kvm][SRU][PATCH 1/1] mm: mlock: avoid increase mm->locked_vm on mlock() when already mlock2(, MLOCK_ONFAULT)

Po-Hsu Lin po-hsu.lin at canonical.com
Tue Oct 23 08:44:52 UTC 2018

From: Simon Guo <wei.guo.simon at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1793451

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:

  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.

Link: http://lkml.kernel.org/r/1472554781-9835-3-git-send-email-wei.guo.simon@gmail.com
Signed-off-by: Simon Guo <wei.guo.simon at gmail.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Cc: Alexey Klimov <klimov.linux at gmail.com>
Cc: Eric B Munson <emunson at akamai.com>
Cc: Geert Uytterhoeven <geert at linux-m68k.org>
Cc: Mel Gorman <mgorman at techsingularity.net>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Shuah Khan <shuah at kernel.org>
Cc: Simon Guo <wei.guo.simon at gmail.com>
Cc: Thierry Reding <treding at nvidia.com>
Cc: Vlastimil Babka <vbabka at suse.cz>
Cc: David Rientjes <rientjes at google.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit b155b4fde5bdde9fed439cd1f5ea07173df2ed31)
Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
 mm/mlock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index 9d2e773..b03f160 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -504,6 +504,7 @@ 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))
@@ -538,6 +539,8 @@ 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;

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