mmap: avoid merging cloned VMAs
Avoid merging a VMA with another VMA which is cloned from the parent process.
The cloned VMA shares the anon_vma lock with the parent process's VMA. If
we do the merge, more vmas (even the new range is only for current
process) use the perent process's anon_vma lock. This introduces
scalability issues. find_mergeable_anon_vma() already considers this
case.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mmap.c b/mm/mmap.c
index 15b1fae..4fb5464 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -703,9 +703,17 @@
}
static inline int is_mergeable_anon_vma(struct anon_vma *anon_vma1,
- struct anon_vma *anon_vma2)
+ struct anon_vma *anon_vma2,
+ struct vm_area_struct *vma)
{
- return !anon_vma1 || !anon_vma2 || (anon_vma1 == anon_vma2);
+ /*
+ * The list_is_singular() test is to avoid merging VMA cloned from
+ * parents. This can improve scalability caused by anon_vma lock.
+ */
+ if ((!anon_vma1 || !anon_vma2) && (!vma ||
+ list_is_singular(&vma->anon_vma_chain)))
+ return 1;
+ return anon_vma1 == anon_vma2;
}
/*
@@ -724,7 +732,7 @@
struct anon_vma *anon_vma, struct file *file, pgoff_t vm_pgoff)
{
if (is_mergeable_vma(vma, file, vm_flags) &&
- is_mergeable_anon_vma(anon_vma, vma->anon_vma)) {
+ is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
if (vma->vm_pgoff == vm_pgoff)
return 1;
}
@@ -743,7 +751,7 @@
struct anon_vma *anon_vma, struct file *file, pgoff_t vm_pgoff)
{
if (is_mergeable_vma(vma, file, vm_flags) &&
- is_mergeable_anon_vma(anon_vma, vma->anon_vma)) {
+ is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
pgoff_t vm_pglen;
vm_pglen = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
if (vma->vm_pgoff + vm_pglen == vm_pgoff)
@@ -821,7 +829,7 @@
can_vma_merge_before(next, vm_flags,
anon_vma, file, pgoff+pglen) &&
is_mergeable_anon_vma(prev->anon_vma,
- next->anon_vma)) {
+ next->anon_vma, NULL)) {
/* cases 1, 6 */
err = vma_adjust(prev, prev->vm_start,
next->vm_end, prev->vm_pgoff, NULL);