> Looking at the if condition, brk() would appear to be the most important
> case, right? This would appear to correlate with the reasoning behind
> that condition in the first place in commit
> 252c5f94d944487e9f50ece7942b0fbf659c5c31 where sbrk contended on the
> lock heavily.
You are right. Here is a new patch 4/5:
---------------------
Subject: always lock the root (oldest) anon_vma
Always (and only) lock the root (oldest) anon_vma whenever we do something in an
anon_vma. The recently introduced anon_vma scalability is due to the rmap code
scanning only the VMAs that need to be scanned. Many common operations still
took the anon_vma lock on the root anon_vma, so always taking that lock is not
expected to introduce any scalability issues.
However, always taking the same lock does mean we only need to take one lock,
which means rmap_walk on pages from any anon_vma in the vma is excluded from
occurring during an munmap, expand_stack or other operation that needs to
exclude rmap_walk and similar functions.
Also add the proper locking to vma_adjust.
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
v2:
- conditionally take the anon_vma lock in vma_adjust, like introduced
in 252c5f94d944487e9f50ece7942b0fbf659c5c31 (with a proper comment)
include/linux/rmap.h | 8 ++++----
mm/ksm.c | 2 +-
mm/mmap.c | 16 +++++++++++++++-
3 files changed, 20 insertions(+), 6 deletions(-)
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 457ae1e..33ffe14 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -95,24 +95,24 @@ static inline void vma_lock_anon_vma(struct vm_area_struct *vma)
{
struct anon_vma *anon_vma = vma->anon_vma;
if (anon_vma)
- spin_lock(&anon_vma->lock);
+ spin_lock(&anon_vma->root->lock);
}
static inline void vma_unlock_anon_vma(struct vm_area_struct *vma)
{
struct anon_vma *anon_vma = vma->anon_vma;
if (anon_vma)
- spin_unlock(&anon_vma->lock);
+ spin_unlock(&anon_vma->root->lock);
}
static inline void anon_vma_lock(struct anon_vma *anon_vma)
{
- spin_lock(&anon_vma->lock);
+ spin_lock(&anon_vma->root->lock);
}
static inline void anon_vma_unlock(struct anon_vma *anon_vma)
{
- spin_unlock(&anon_vma->lock);
+ spin_unlock(&anon_vma->root->lock);
}
/*
diff --git a/mm/ksm.c b/mm/ksm.c
index d488012..7ca0dd7 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -325,7 +325,7 @@ static void drop_anon_vma(struct rmap_item *rmap_item)
{
struct anon_vma *anon_vma = rmap_item->anon_vma;
- if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->lock)) {
+ if (atomic_dec_and_lock(&anon_vma->ksm_refcount, &anon_vma->root->lock)) {
int empty = list_empty(&anon_vma->head);
anon_vma_unlock(anon_vma);
if (empty)
diff --git a/mm/mmap.c b/mm/mmap.c
index f70bc65..a543359 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -506,6 +506,7 @@ int vma_adjust(struct vm_area_struct *vma, unsigned long start,
struct vm_area_struct *importer = NULL;
struct address_space *mapping = NULL;
struct prio_tree_root *root = NULL;
+ struct anon_vma *anon_vma = NULL;
struct file *file = vma->vm_file;
long adjust_next = 0;
int remove_next = 0;
@@ -553,6 +554,17 @@ again: remove_next = 1 + (end > next->vm_end);
}
}
+ /*
+ * When changing only vma->vm_end, we don't really need anon_vma
+ * lock. This is a fairly rare case by itself, but the anon_vma
+ * lock may be shared between many sibling processes. Skipping
+ * the lock for brk adjustments makes a difference sometimes.
+ */
+ if (vma->anon_vma && (insert || importer || start != vma->vm_start)) {
+ anon_vma = vma->anon_vma;
+ anon_vma_lock(anon_vma);
+ }
+
if (file) {
mapping = file->f_mapping;
if (!(vma->vm_flags & VM_NONLINEAR))
@@ -619,6 +631,8 @@ again: remove_next = 1 + (end > next->vm_end);
if (mapping)
spin_unlock(&mapping->i_mmap_lock);
+ if (anon_vma)
+ anon_vma_unlock(anon_vma);
if (remove_next) {
if (file) {
@@ -2471,7 +2485,7 @@ static void vm_lock_anon_vma(struct mm_struct *mm, struct anon_vma *anon_vma)
* The LSB of head.next can't change from under us
* because we hold the mm_all_locks_mutex.
*/
- spin_lock_nest_lock(&anon_vma->lock, &mm->mmap_sem);
+ spin_lock_nest_lock(&anon_vma->root->lock, &mm->mmap_sem);
/*
* We can safely modify head.next after taking the
* anon_vma->lock. If some other vma in this mm shares
--
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