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Date: Tue, 4 Sep 2012 02:20:57 -0700
From: Michel Lespinasse <walken@...gle.com>
To: linux-mm@...ck.org, riel@...hat.com, peterz@...radead.org,
aarcange@...hat.com, hughd@...gle.com, daniel.santos@...ox.com
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: [PATCH 7/7] 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@...gle.com>
---
fs/exec.c | 2 +-
include/linux/mm.h | 6 +++-
mm/mmap.c | 7 ++++-
mm/mremap.c | 57 +++++++++++++++++++++++++++++++++++----------------
4 files changed, 49 insertions(+), 23 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index da27b91ff1e8..f7b4009ea327 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -613,7 +613,7 @@ static int shift_arg_pages(struct vm_area_struct *vma, unsigned long shift)
* process cleanup to remove whatever mess we made.
*/
if (length != move_page_tables(vma, old_start,
- vma, new_start, length))
+ vma, new_start, length, false))
return -ENOMEM;
lru_add_drain();
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1a2b1a44bd4e..c6a6b0b0f176 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1042,7 +1042,8 @@ vm_is_stack(struct task_struct *task, struct vm_area_struct *vma, int in_group);
extern 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);
extern unsigned long do_mremap(unsigned long addr,
unsigned long old_len, unsigned long new_len,
unsigned long flags, unsigned long new_addr);
@@ -1391,7 +1392,8 @@ extern void __vma_link_rb(struct mm_struct *, struct vm_area_struct *,
struct rb_node **, struct rb_node *);
extern void unlink_file_vma(struct vm_area_struct *);
extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
- unsigned long addr, unsigned long len, pgoff_t pgoff);
+ unsigned long addr, unsigned long len, pgoff_t pgoff,
+ bool *need_rmap_locks);
extern void exit_mmap(struct mm_struct *);
extern int mm_take_all_locks(struct mm_struct *mm);
diff --git a/mm/mmap.c b/mm/mmap.c
index 884bda4cd3ea..cc8c64077a42 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2397,7 +2397,8 @@ int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)
* prior to moving page table entries, to effect an mremap move.
*/
struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
- unsigned long addr, unsigned long len, pgoff_t pgoff)
+ unsigned long addr, unsigned long len, pgoff_t pgoff,
+ bool *need_rmap_locks)
{
struct vm_area_struct *vma = *vmap;
unsigned long vma_start = vma->vm_start;
@@ -2438,8 +2439,9 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
* linear if there are no pages mapped yet.
*/
VM_BUG_ON(faulted_in_anon_vma);
- *vmap = new_vma;
+ *vmap = vma = new_vma;
}
+ *need_rmap_locks = (new_vma->vm_pgoff <= vma->vm_pgoff);
} else {
new_vma = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);
if (new_vma) {
@@ -2466,6 +2468,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
if (new_vma->vm_ops && new_vma->vm_ops->open)
new_vma->vm_ops->open(new_vma);
vma_link(mm, new_vma, prev, rb_link, rb_parent);
+ *need_rmap_locks = false;
}
}
return new_vma;
diff --git a/mm/mremap.c b/mm/mremap.c
index 95fb2e024ced..d18cdf09868c 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -71,26 +71,42 @@ static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
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 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
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 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
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 @@ static unsigned long move_vma(struct vm_area_struct *vma,
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 @@ static unsigned long move_vma(struct vm_area_struct *vma,
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;
--
1.7.7.3
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