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Subject[PATCH 2/2] mm/migration: Make rmap_walk_anon() and try_to_unmap_anon() more scalable

Note, with this optimization I went a farther than the
boundaries of the migration code - it seemed worthwile to do and
I've reviewed all the other users of page_lock_anon_vma() as
well and none seemed to be modifying the list inside that lock.

Please review this patch carefully - in particular the SMP races
outlined in anon_vma_free() are exciting: I have updated the
reasoning and it still appears to hold, but please double check
the changes nevertheless ...

Thanks,

Ingo

------------------->
From: Ingo Molnar <mingo@kernel.org>
Date: Sat Dec 1 20:43:04 CET 2012

rmap_walk_anon() and try_to_unmap_anon() appears to be too careful
about locking the anon vma: while it needs protection against anon
vma list modifications, it does not need exclusive access to the
list itself.

Transforming this exclusive lock to a read-locked rwsem removes a
global lock from the hot path of page-migration intense threaded
workloads which can cause pathological performance like this:

96.43% process 0 [kernel.kallsyms] [k] perf_trace_sched_switch
|
--- perf_trace_sched_switch
__schedule
schedule
schedule_preempt_disabled
__mutex_lock_common.isra.6
__mutex_lock_slowpath
mutex_lock
|
|--50.61%-- rmap_walk
| move_to_new_page
| migrate_pages
| migrate_misplaced_page
| __do_numa_page.isra.69
| handle_pte_fault
| handle_mm_fault
| __do_page_fault
| do_page_fault
| page_fault
| __memset_sse2
| |
| --100.00%-- worker_thread
| |
| --100.00%-- start_thread
|
--49.39%-- page_lock_anon_vma
try_to_unmap_anon
try_to_unmap
migrate_pages
migrate_misplaced_page
__do_numa_page.isra.69
handle_pte_fault
handle_mm_fault
__do_page_fault
do_page_fault
page_fault
__memset_sse2
|
--100.00%-- worker_thread
start_thread

With this change applied the profile is now nicely flat
and there's no anon-vma related scheduling/blocking.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/linux/rmap.h | 15 +++++++++++++--
mm/huge_memory.c | 4 ++--
mm/memory-failure.c | 4 ++--
mm/migrate.c | 2 +-
mm/rmap.c | 40 ++++++++++++++++++++--------------------
5 files changed, 38 insertions(+), 27 deletions(-)

Index: linux/include/linux/rmap.h
===================================================================
--- linux.orig/include/linux/rmap.h
+++ linux/include/linux/rmap.h
@@ -128,6 +128,17 @@ static inline void anon_vma_unlock(struc
up_write(&anon_vma->root->rwsem);
}

+static inline void anon_vma_lock_read(struct anon_vma *anon_vma)
+{
+ down_read(&anon_vma->root->rwsem);
+}
+
+static inline void anon_vma_unlock_read(struct anon_vma *anon_vma)
+{
+ up_read(&anon_vma->root->rwsem);
+}
+
+
/*
* anon_vma helper functions.
*/
@@ -220,8 +231,8 @@ int try_to_munlock(struct page *);
/*
* Called by memory-failure.c to kill processes.
*/
-struct anon_vma *page_lock_anon_vma(struct page *page);
-void page_unlock_anon_vma(struct anon_vma *anon_vma);
+struct anon_vma *page_lock_anon_vma_read(struct page *page);
+void page_unlock_anon_vma_read(struct anon_vma *anon_vma);
int page_mapped_in_vma(struct page *page, struct vm_area_struct *vma);

/*
Index: linux/mm/huge_memory.c
===================================================================
--- linux.orig/mm/huge_memory.c
+++ linux/mm/huge_memory.c
@@ -1645,7 +1645,7 @@ int split_huge_page(struct page *page)
int ret = 1;

BUG_ON(!PageAnon(page));
- anon_vma = page_lock_anon_vma(page);
+ anon_vma = page_lock_anon_vma_read(page);
if (!anon_vma)
goto out;
ret = 0;
@@ -1658,7 +1658,7 @@ int split_huge_page(struct page *page)

BUG_ON(PageCompound(page));
out_unlock:
- page_unlock_anon_vma(anon_vma);
+ page_unlock_anon_vma_read(anon_vma);
out:
return ret;
}
Index: linux/mm/memory-failure.c
===================================================================
--- linux.orig/mm/memory-failure.c
+++ linux/mm/memory-failure.c
@@ -402,7 +402,7 @@ static void collect_procs_anon(struct pa
struct anon_vma *av;
pgoff_t pgoff;

- av = page_lock_anon_vma(page);
+ av = page_lock_anon_vma_read(page);
if (av == NULL) /* Not actually mapped anymore */
return;

@@ -423,7 +423,7 @@ static void collect_procs_anon(struct pa
}
}
read_unlock(&tasklist_lock);
- page_unlock_anon_vma(av);
+ page_unlock_anon_vma_read(av);
}

/*
Index: linux/mm/migrate.c
===================================================================
--- linux.orig/mm/migrate.c
+++ linux/mm/migrate.c
@@ -751,7 +751,7 @@ static int __unmap_and_move(struct page
*/
if (PageAnon(page)) {
/*
- * Only page_lock_anon_vma() understands the subtleties of
+ * Only page_lock_anon_vma_read() understands the subtleties of
* getting a hold on an anon_vma from outside one of its mms.
*/
anon_vma = page_get_anon_vma(page);
Index: linux/mm/rmap.c
===================================================================
--- linux.orig/mm/rmap.c
+++ linux/mm/rmap.c
@@ -87,18 +87,18 @@ static inline void anon_vma_free(struct
VM_BUG_ON(atomic_read(&anon_vma->refcount));

/*
- * Synchronize against page_lock_anon_vma() such that
+ * Synchronize against page_lock_anon_vma_read() such that
* we can safely hold the lock without the anon_vma getting
* freed.
*
* Relies on the full mb implied by the atomic_dec_and_test() from
* put_anon_vma() against the acquire barrier implied by
- * mutex_trylock() from page_lock_anon_vma(). This orders:
+ * down_read_trylock() from page_lock_anon_vma_read(). This orders:
*
- * page_lock_anon_vma() VS put_anon_vma()
- * mutex_trylock() atomic_dec_and_test()
+ * page_lock_anon_vma_read() VS put_anon_vma()
+ * down_read_trylock() atomic_dec_and_test()
* LOCK MB
- * atomic_read() mutex_is_locked()
+ * atomic_read() rwsem_is_locked()
*
* LOCK should suffice since the actual taking of the lock must
* happen _before_ what follows.
@@ -146,7 +146,7 @@ static void anon_vma_chain_link(struct v
* allocate a new one.
*
* Anon-vma allocations are very subtle, because we may have
- * optimistically looked up an anon_vma in page_lock_anon_vma()
+ * optimistically looked up an anon_vma in page_lock_anon_vma_read()
* and that may actually touch the spinlock even in the newly
* allocated vma (it depends on RCU to make sure that the
* anon_vma isn't actually destroyed).
@@ -442,7 +442,7 @@ out:
* atomic op -- the trylock. If we fail the trylock, we fall back to getting a
* reference like with page_get_anon_vma() and then block on the mutex.
*/
-struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *page_lock_anon_vma_read(struct page *page)
{
struct anon_vma *anon_vma = NULL;
struct anon_vma *root_anon_vma;
@@ -457,14 +457,14 @@ struct anon_vma *page_lock_anon_vma(stru

anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
root_anon_vma = ACCESS_ONCE(anon_vma->root);
- if (down_write_trylock(&root_anon_vma->rwsem)) {
+ if (down_read_trylock(&root_anon_vma->rwsem)) {
/*
* If the page is still mapped, then this anon_vma is still
* its anon_vma, and holding the mutex ensures that it will
* not go away, see anon_vma_free().
*/
if (!page_mapped(page)) {
- up_write(&root_anon_vma->rwsem);
+ up_read(&root_anon_vma->rwsem);
anon_vma = NULL;
}
goto out;
@@ -484,7 +484,7 @@ struct anon_vma *page_lock_anon_vma(stru

/* we pinned the anon_vma, its safe to sleep */
rcu_read_unlock();
- anon_vma_lock(anon_vma);
+ anon_vma_lock_read(anon_vma);

if (atomic_dec_and_test(&anon_vma->refcount)) {
/*
@@ -492,7 +492,7 @@ struct anon_vma *page_lock_anon_vma(stru
* and bail -- can't simply use put_anon_vma() because
* we'll deadlock on the anon_vma_lock() recursion.
*/
- anon_vma_unlock(anon_vma);
+ anon_vma_unlock_read(anon_vma);
__put_anon_vma(anon_vma);
anon_vma = NULL;
}
@@ -504,9 +504,9 @@ out:
return anon_vma;
}

-void page_unlock_anon_vma(struct anon_vma *anon_vma)
+void page_unlock_anon_vma_read(struct anon_vma *anon_vma)
{
- anon_vma_unlock(anon_vma);
+ anon_vma_unlock_read(anon_vma);
}

/*
@@ -732,7 +732,7 @@ static int page_referenced_anon(struct p
struct anon_vma_chain *avc;
int referenced = 0;

- anon_vma = page_lock_anon_vma(page);
+ anon_vma = page_lock_anon_vma_read(page);
if (!anon_vma)
return referenced;

@@ -754,7 +754,7 @@ static int page_referenced_anon(struct p
break;
}

- page_unlock_anon_vma(anon_vma);
+ page_unlock_anon_vma_read(anon_vma);
return referenced;
}

@@ -1474,7 +1474,7 @@ static int try_to_unmap_anon(struct page
struct anon_vma_chain *avc;
int ret = SWAP_AGAIN;

- anon_vma = page_lock_anon_vma(page);
+ anon_vma = page_lock_anon_vma_read(page);
if (!anon_vma)
return ret;

@@ -1501,7 +1501,7 @@ static int try_to_unmap_anon(struct page
break;
}

- page_unlock_anon_vma(anon_vma);
+ page_unlock_anon_vma_read(anon_vma);
return ret;
}

@@ -1696,7 +1696,7 @@ static int rmap_walk_anon(struct page *p
int ret = SWAP_AGAIN;

/*
- * Note: remove_migration_ptes() cannot use page_lock_anon_vma()
+ * Note: remove_migration_ptes() cannot use page_lock_anon_vma_read()
* because that depends on page_mapped(); but not all its usages
* are holding mmap_sem. Users without mmap_sem are required to
* take a reference count to prevent the anon_vma disappearing
@@ -1704,7 +1704,7 @@ static int rmap_walk_anon(struct page *p
anon_vma = page_anon_vma(page);
if (!anon_vma)
return ret;
- anon_vma_lock(anon_vma);
+ anon_vma_lock_read(anon_vma);
anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
struct vm_area_struct *vma = avc->vma;
unsigned long address = vma_address(page, vma);
@@ -1712,7 +1712,7 @@ static int rmap_walk_anon(struct page *p
if (ret != SWAP_AGAIN)
break;
}
- anon_vma_unlock(anon_vma);
+ anon_vma_unlock_read(anon_vma);
return ret;
}


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