mm: avoid taking rmap locks in move_ptes()
During mremap(), the destination VMA is generally placed after the
original vma in rmap traversal order: in move_vma(), we always have
new_pgoff >= vma->vm_pgoff, and as a result new_vma->vm_pgoff >=
vma->vm_pgoff unless vma_merge() merged the new vma with an adjacent one.
When the destination VMA is placed after the original in rmap traversal
order, we can avoid taking the rmap locks in move_ptes().
Essentially, this reintroduces the optimization that had been disabled in
"mm anon rmap: remove anon_vma_moveto_tail". The difference is that we
don't try to impose the rmap traversal order; instead we just rely on
things being in the desired order in the common case and fall back to
taking locks in the uncommon case. Also we skip the i_mmap_mutex in
addition to the anon_vma lock: in both cases, the vmas are traversed in
increasing vm_pgoff order with ties resolved in tree insertion order.
Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Daniel Santos <daniel.santos@pobox.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/mremap.c b/mm/mremap.c
index 5588bb6..3b639a4 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -71,26 +71,42 @@
static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
unsigned long old_addr, unsigned long old_end,
struct vm_area_struct *new_vma, pmd_t *new_pmd,
- unsigned long new_addr)
+ unsigned long new_addr, bool need_rmap_locks)
{
struct address_space *mapping = NULL;
- struct anon_vma *anon_vma = vma->anon_vma;
+ struct anon_vma *anon_vma = NULL;
struct mm_struct *mm = vma->vm_mm;
pte_t *old_pte, *new_pte, pte;
spinlock_t *old_ptl, *new_ptl;
- if (vma->vm_file) {
- /*
- * Subtle point from Rajesh Venkatasubramanian: before
- * moving file-based ptes, we must lock truncate_pagecache
- * out, since it might clean the dst vma before the src vma,
- * and we propagate stale pages into the dst afterward.
- */
- mapping = vma->vm_file->f_mapping;
- mutex_lock(&mapping->i_mmap_mutex);
+ /*
+ * When need_rmap_locks is true, we take the i_mmap_mutex and anon_vma
+ * locks to ensure that rmap will always observe either the old or the
+ * new ptes. This is the easiest way to avoid races with
+ * truncate_pagecache(), page migration, etc...
+ *
+ * When need_rmap_locks is false, we use other ways to avoid
+ * such races:
+ *
+ * - During exec() shift_arg_pages(), we use a specially tagged vma
+ * which rmap call sites look for using is_vma_temporary_stack().
+ *
+ * - During mremap(), new_vma is often known to be placed after vma
+ * in rmap traversal order. This ensures rmap will always observe
+ * either the old pte, or the new pte, or both (the page table locks
+ * serialize access to individual ptes, but only rmap traversal
+ * order guarantees that we won't miss both the old and new ptes).
+ */
+ if (need_rmap_locks) {
+ if (vma->vm_file) {
+ mapping = vma->vm_file->f_mapping;
+ mutex_lock(&mapping->i_mmap_mutex);
+ }
+ if (vma->anon_vma) {
+ anon_vma = vma->anon_vma;
+ anon_vma_lock(anon_vma);
+ }
}
- if (anon_vma)
- anon_vma_lock(anon_vma);
/*
* We don't have to worry about the ordering of src and dst
@@ -127,7 +143,8 @@
unsigned long move_page_tables(struct vm_area_struct *vma,
unsigned long old_addr, struct vm_area_struct *new_vma,
- unsigned long new_addr, unsigned long len)
+ unsigned long new_addr, unsigned long len,
+ bool need_rmap_locks)
{
unsigned long extent, next, old_end;
pmd_t *old_pmd, *new_pmd;
@@ -174,7 +191,7 @@
if (extent > LATENCY_LIMIT)
extent = LATENCY_LIMIT;
move_ptes(vma, old_pmd, old_addr, old_addr + extent,
- new_vma, new_pmd, new_addr);
+ new_vma, new_pmd, new_addr, need_rmap_locks);
need_flush = true;
}
if (likely(need_flush))
@@ -198,6 +215,7 @@
unsigned long hiwater_vm;
int split = 0;
int err;
+ bool need_rmap_locks;
/*
* We'd prefer to avoid failure later on in do_munmap:
@@ -219,18 +237,21 @@
return err;
new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
- new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff);
+ new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
+ &need_rmap_locks);
if (!new_vma)
return -ENOMEM;
- moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len);
+ moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
+ need_rmap_locks);
if (moved_len < old_len) {
/*
* On error, move entries back from new area to old,
* which will succeed since page tables still there,
* and then proceed to unmap new area instead of old.
*/
- move_page_tables(new_vma, new_addr, vma, old_addr, moved_len);
+ move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
+ true);
vma = new_vma;
old_len = new_len;
old_addr = new_addr;