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Subject[PATCH] rmaplock 4/6 mm lock ordering
With page_map_lock out of the way, there's no need for page_referenced
and try_to_unmap to use trylocks - provided we switch anon_vma->lock and
mm->page_table_lock around in anon_vma_prepare. Though I suppose it's
possible that we'll find that vmscan makes better progress with trylocks
than spinning - we're free to choose trylocks again if so.

Try to update the mm lock ordering documentation in filemap.c.
But I still find it confusing, and I've no idea of where to stop.
So add the mm lock ordering list I can understand to rmap.c.

Signed-off-by: Hugh Dickins <hugh@veritas.com>

mm/filemap.c | 15 ++++++----
mm/rmap.c | 82 ++++++++++++++++++++++++++++++++---------------------------
2 files changed, 54 insertions(+), 43 deletions(-)
--- rmaplock3/mm/filemap.c 2004-07-09 10:53:46.000000000 +0100
+++ rmaplock4/mm/filemap.c 2004-07-12 18:20:48.023890560 +0100
@@ -60,7 +60,6 @@
* ->swap_list_lock
* ->swap_device_lock (exclusive_swap_page, others)
* ->mapping->tree_lock
- * ->page_map_lock() (try_to_unmap_file)
*
* ->i_sem
* ->i_mmap_lock (truncate->unmap_mapping_range)
@@ -83,16 +82,20 @@
* ->sb_lock (fs/fs-writeback.c)
* ->mapping->tree_lock (__sync_single_inode)
*
+ * ->i_mmap_lock
+ * ->anon_vma.lock (vma_adjust)
+ *
+ * ->anon_vma.lock
+ * ->page_table_lock (anon_vma_prepare and various)
+ *
* ->page_table_lock
* ->swap_device_lock (try_to_unmap_one)
* ->private_lock (try_to_unmap_one)
* ->tree_lock (try_to_unmap_one)
* ->zone.lru_lock (follow_page->mark_page_accessed)
- * ->page_map_lock() (page_add_anon_rmap)
- * ->tree_lock (page_remove_rmap->set_page_dirty)
- * ->private_lock (page_remove_rmap->set_page_dirty)
- * ->inode_lock (page_remove_rmap->set_page_dirty)
- * ->anon_vma.lock (anon_vma_prepare)
+ * ->private_lock (page_remove_rmap->set_page_dirty)
+ * ->tree_lock (page_remove_rmap->set_page_dirty)
+ * ->inode_lock (page_remove_rmap->set_page_dirty)
* ->inode_lock (zap_pte_range->set_page_dirty)
* ->private_lock (zap_pte_range->__set_page_dirty_buffers)
*
--- rmaplock3/mm/rmap.c 2004-07-12 18:20:35.274828712 +0100
+++ rmaplock4/mm/rmap.c 2004-07-12 18:20:48.025890256 +0100
@@ -18,9 +18,30 @@
*/

/*
- * Locking: see "Lock ordering" summary in filemap.c.
- * In swapout, page_map_lock is held on entry to page_referenced and
- * try_to_unmap, so they trylock for i_mmap_lock and page_table_lock.
+ * Lock ordering in mm:
+ *
+ * inode->i_sem (while writing or truncating, not reading or faulting)
+ * inode->i_alloc_sem
+ *
+ * When a page fault occurs in writing from user to file, down_read
+ * of mmap_sem nests within i_sem; in sys_msync, i_sem nests within
+ * down_read of mmap_sem; i_sem and down_write of mmap_sem are never
+ * taken together; in truncation, i_sem is taken outermost.
+ *
+ * mm->mmap_sem
+ * page->flags PG_locked (lock_page)
+ * mapping->i_mmap_lock
+ * anon_vma->lock
+ * mm->page_table_lock
+ * zone->lru_lock (in mark_page_accessed)
+ * swap_list_lock (in swap_free etc's swap_info_get)
+ * swap_device_lock (in swap_duplicate, swap_info_get)
+ * mapping->private_lock (in __set_page_dirty_buffers)
+ * inode_lock (in set_page_dirty's __mark_inode_dirty)
+ * sb_lock (within inode_lock in fs/fs-writeback.c)
+ * mapping->tree_lock (widely used, in set_page_dirty,
+ * in arch-dependent flush_dcache_mmap_lock,
+ * within inode_lock in __sync_single_inode)
*/

#include <linux/mm.h>
@@ -64,28 +85,32 @@ int anon_vma_prepare(struct vm_area_stru
might_sleep();
if (unlikely(!anon_vma)) {
struct mm_struct *mm = vma->vm_mm;
- struct anon_vma *allocated = NULL;
+ struct anon_vma *allocated, *locked;

anon_vma = find_mergeable_anon_vma(vma);
- if (!anon_vma) {
+ if (anon_vma) {
+ allocated = NULL;
+ locked = anon_vma;
+ spin_lock(&locked->lock);
+ } else {
anon_vma = anon_vma_alloc();
if (unlikely(!anon_vma))
return -ENOMEM;
allocated = anon_vma;
+ locked = NULL;
}

/* page_table_lock to protect against threads */
spin_lock(&mm->page_table_lock);
if (likely(!vma->anon_vma)) {
- if (!allocated)
- spin_lock(&anon_vma->lock);
vma->anon_vma = anon_vma;
list_add(&vma->anon_vma_node, &anon_vma->head);
- if (!allocated)
- spin_unlock(&anon_vma->lock);
allocated = NULL;
}
spin_unlock(&mm->page_table_lock);
+
+ if (locked)
+ spin_unlock(&locked->lock);
if (unlikely(allocated))
anon_vma_free(allocated);
}
@@ -225,8 +250,7 @@ static int page_referenced_one(struct pa
if (address == -EFAULT)
goto out;

- if (!spin_trylock(&mm->page_table_lock))
- goto out;
+ spin_lock(&mm->page_table_lock);

pgd = pgd_offset(mm, address);
if (!pgd_present(*pgd))
@@ -287,9 +311,6 @@ static int page_referenced_anon(struct p
* of references it found.
*
* This function is only called from page_referenced for object-based pages.
- *
- * The spinlock address_space->i_mmap_lock is tried. If it can't be gotten,
- * assume a reference count of 0, so try_to_unmap will then have a go.
*/
static int page_referenced_file(struct page *page)
{
@@ -315,8 +336,7 @@ static int page_referenced_file(struct p
*/
BUG_ON(!PageLocked(page));

- if (!spin_trylock(&mapping->i_mmap_lock))
- return 0;
+ spin_lock(&mapping->i_mmap_lock);

/*
* i_mmap_lock does not stabilize mapcount at all, but mapcount
@@ -468,8 +488,7 @@ static int try_to_unmap_one(struct page
* We need the page_table_lock to protect us from page faults,
* munmap, fork, etc...
*/
- if (!spin_trylock(&mm->page_table_lock))
- goto out;
+ spin_lock(&mm->page_table_lock);

pgd = pgd_offset(mm, address);
if (!pgd_present(*pgd))
@@ -568,7 +587,7 @@ out:
#define CLUSTER_SIZE min(32*PAGE_SIZE, PMD_SIZE)
#define CLUSTER_MASK (~(CLUSTER_SIZE - 1))

-static int try_to_unmap_cluster(unsigned long cursor,
+static void try_to_unmap_cluster(unsigned long cursor,
unsigned int *mapcount, struct vm_area_struct *vma)
{
struct mm_struct *mm = vma->vm_mm;
@@ -585,8 +604,7 @@ static int try_to_unmap_cluster(unsigned
* We need the page_table_lock to protect us from page faults,
* munmap, fork, etc...
*/
- if (!spin_trylock(&mm->page_table_lock))
- return SWAP_FAIL;
+ spin_lock(&mm->page_table_lock);

address = (vma->vm_start + cursor) & CLUSTER_MASK;
end = address + CLUSTER_SIZE;
@@ -643,7 +661,6 @@ static int try_to_unmap_cluster(unsigned

out_unlock:
spin_unlock(&mm->page_table_lock);
- return SWAP_AGAIN;
}

static int try_to_unmap_anon(struct page *page)
@@ -673,9 +690,6 @@ static int try_to_unmap_anon(struct page
* contained in the address_space struct it points to.
*
* This function is only called from try_to_unmap for object-based pages.
- *
- * The spinlock address_space->i_mmap_lock is tried. If it can't be gotten,
- * return a temporary error.
*/
static int try_to_unmap_file(struct page *page)
{
@@ -689,9 +703,7 @@ static int try_to_unmap_file(struct page
unsigned long max_nl_size = 0;
unsigned int mapcount;

- if (!spin_trylock(&mapping->i_mmap_lock))
- return ret;
-
+ spin_lock(&mapping->i_mmap_lock);
while ((vma = vma_prio_tree_next(vma, &mapping->i_mmap,
&iter, pgoff, pgoff)) != NULL) {
ret = try_to_unmap_one(page, vma);
@@ -714,8 +726,10 @@ static int try_to_unmap_file(struct page
max_nl_size = cursor;
}

- if (max_nl_size == 0) /* any nonlinears locked or reserved */
+ if (max_nl_size == 0) { /* any nonlinears locked or reserved */
+ ret = SWAP_FAIL;
goto out;
+ }

/*
* We don't try to search for this page in the nonlinear vmas,
@@ -742,19 +756,13 @@ static int try_to_unmap_file(struct page
while (vma->vm_mm->rss &&
cursor < max_nl_cursor &&
cursor < vma->vm_end - vma->vm_start) {
- ret = try_to_unmap_cluster(
- cursor, &mapcount, vma);
- if (ret == SWAP_FAIL)
- break;
+ try_to_unmap_cluster(cursor, &mapcount, vma);
cursor += CLUSTER_SIZE;
vma->vm_private_data = (void *) cursor;
if ((int)mapcount <= 0)
goto out;
}
- if (ret != SWAP_FAIL)
- vma->vm_private_data =
- (void *) max_nl_cursor;
- ret = SWAP_AGAIN;
+ vma->vm_private_data = (void *) max_nl_cursor;
}
cond_resched_lock(&mapping->i_mmap_lock);
max_nl_cursor += CLUSTER_SIZE;
-
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