improve sync efficiency with sb inode wb list

From: Dave Chinner <[email protected]>

wait_sb_inodes() currently does a walk of all inodes in the filesystem to find dirty one to wait on during sync. This is highly inefficient and wastes a lot of CPU when there are lots of clean cached inodes that we don't need to wait on.

To avoid this "all inode" walk, we need to track inodes that are currently under writeback that we need to wait for. We do this by adding inodes to a writeback list on the sb when the mapping is first tagged as having pages under writeback. wait_sb_inodes() can then walk this list of "inodes under IO" and wait specifically just for the inodes that the current sync(2) needs to wait for.

Define a couple helpers to add/remove an inode from the writeback list and call them when the overall mapping is tagged for or cleared from writeback. Update wait_sb_inodes() to walk only the inodes under writeback due to the sync.

Signed-off-by Dave Chinner <[email protected]>

Signed-off-by Josef Bacik <[email protected]>

Signed-off-by Brian Foster <[email protected]>

---
 fs/block_dev.c            |   1 +
 fs/fs-writeback.c         | 139 +++++++++++++++++++++++++++++++++++++---------
 fs/inode.c                |   1 +
 fs/super.c                |   2 +
 include/linux/fs.h        |   4 ++
 include/linux/writeback.h |   3 +
 mm/page-writeback.c       |  19 +++++++
 7 files changed, 144 insertions(+), 25 deletions(-)

fs/fs-writeback.c +0 -0

index 023f6a1..6821347 100644

fs/fs-writeback.c +4 -1

@@ -1118,13 +1149,28 @@ static void __inode_wait_for_writeback(struct inode *inode)
 }
 
 /*
- * Wait for writeback on an inode to complete. Caller must have inode pinned.
+ * Wait for writeback on an inode to complete during eviction. Caller must have
+ * inode pinned.
  */
 void inode_wait_for_writeback(struct inode *inode)
 {
+	BUG_ON(!(inode->i_state & I_FREEING));
+
 	spin_lock(&inode->i_lock);
 	__inode_wait_for_writeback(inode);

fs/fs-writeback.c +70 -24

+	if (!sb_is_blkdev_sb(inode->i_sb))
+		sb_clear_inode_writeback(inode);
+	BUG_ON(!list_empty(&inode->i_wb_list));
 }
 
 /*

@@ -2108,7 +2154,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
  */
 static void wait_sb_inodes(struct super_block *sb)
 {
-	struct inode *inode, *old_inode = NULL;
+	LIST_HEAD(sync_list);
 
 	/*
 	 * We need to be protected against the filesystem going from

@@ -2116,23 +2162,56 @@ static void wait_sb_inodes(struct super_block *sb)
 	 */
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback, because
+	 * there may have been pages dirtied before our sync call, but which had
+	 * writeout started before we write it out.  In which case, the inode
+	 * may not be on the dirty list, but we still have to wait for that
+	 * writeout.
+	 *
+	 * To avoid syncing inodes put under IO after we have started here,
+	 * splice the io list to a temporary list head and walk that. Newly
+	 * dirtied inodes will go onto the primary list so we won't wait for
+	 * them. This is safe to do as we are serialised by the s_sync_lock,
+	 * so we'll complete processing the complete list before the next
+	 * sync operation repeats the splice-and-walk process.
+	 *
+	 * s_inode_wblist_lock protects the wb list and is irq-safe as it is
+	 * acquired inside of the mapping lock by __test_set_page_writeback().
+	 * We cannot acquire i_lock while the wblist lock is held without
+	 * introducing irq inversion issues. Since s_inodes_wb is a subset of
+	 * s_inodes, use s_inode_list_lock to prevent inodes from disappearing
+	 * until we have a reference. Note that s_inode_wblist_lock protects the
+	 * local sync_list as well because inodes can be dropped from either
+	 * list by writeback completion.
+	 */
 	mutex_lock(&sb->s_sync_lock);
+
 	spin_lock(&sb->s_inode_list_lock);
+	spin_lock_irq(&sb->s_inode_wblist_lock);
+	list_splice_init(&sb->s_inodes_wb, &sync_list);
 
-	/*
-	 * Data integrity sync. Must wait for all pages under writeback,
-	 * because there may have been pages dirtied before our sync
-	 * call, but which had writeout started before we write it out.
-	 * In which case, the inode may not be on the dirty list, but
-	 * we still have to wait for that writeout.
-	 */
-	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+	while (!list_empty(&sync_list)) {
+		struct inode *inode = list_first_entry(&sync_list, struct inode,
+						       i_wb_list);
 		struct address_space *mapping = inode->i_mapping;
 
+		list_del_init(&inode->i_wb_list);
+
+		/*
+		 * The mapping can be cleaned before we wait on the inode since
+		 * we do not have the mapping lock.
+		 */
+		if (!mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK))
+			continue;
+
+		spin_unlock_irq(&sb->s_inode_wblist_lock);
+
 		spin_lock(&inode->i_lock);
-		if ((inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) ||
-		    (mapping->nrpages == 0)) {
+		if (inode->i_state & (I_FREEING|I_WILL_FREE|I_NEW)) {
 			spin_unlock(&inode->i_lock);
+
+			spin_lock_irq(&sb->s_inode_wblist_lock);
 			continue;
 		}
 		__iget(inode);

@@ -2140,29 +2219,39 @@ static void wait_sb_inodes(struct super_block *sb)
 		spin_unlock(&sb->s_inode_list_lock);
 
 		/*
-		 * We hold a reference to 'inode' so it couldn't have been
-		 * removed from s_inodes list while we dropped the
-		 * s_inode_list_lock.  We cannot iput the inode now as we can
-		 * be holding the last reference and we cannot iput it under
-		 * s_inode_list_lock. So we keep the reference and iput it
-		 * later.
-		 */
-		iput(old_inode);
-		old_inode = inode;
-
-		/*
 		 * We keep the error status of individual mapping so that
 		 * applications can catch the writeback error using fsync(2).
 		 * See filemap_fdatawait_keep_errors() for details.
 		 */
 		filemap_fdatawait_keep_errors(mapping);
-
 		cond_resched();
 
+		/*
+		 * The inode has been written back. Check whether we still have
+		 * pages under I/O and move the inode back to the primary list
+		 * if necessary. sb_mark_inode_writeback() might not have done
+		 * anything if the writeback tag hadn't been cleared from the
+		 * mapping by the time more wb had started.
+		 */
+		spin_lock_irq(&mapping->tree_lock);
+		if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
+			spin_lock(&sb->s_inode_wblist_lock);
+			if (list_empty(&inode->i_wb_list))
+				list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
+			spin_unlock(&sb->s_inode_wblist_lock);
+		} else
+			WARN_ON(!list_empty(&inode->i_wb_list));
+		spin_unlock_irq(&mapping->tree_lock);
+
+		iput(inode);
+
 		spin_lock(&sb->s_inode_list_lock);
+		spin_lock_irq(&sb->s_inode_wblist_lock);
 	}
+
+	spin_unlock_irq(&sb->s_inode_wblist_lock);
 	spin_unlock(&sb->s_inode_list_lock);
-	iput(old_inode);
+
 	mutex_unlock(&sb->s_sync_lock);
 }
 

fs/inode.c +1 -0

index 5bb85a0..c7a9581 100644
@@ -358,6 +358,7 @@ void inode_init_once(struct inode *inode)
 	INIT_HLIST_NODE(&inode->i_hash);
 	INIT_LIST_HEAD(&inode->i_devices);
 	INIT_LIST_HEAD(&inode->i_io_list);
+	INIT_LIST_HEAD(&inode->i_wb_list);
 	INIT_LIST_HEAD(&inode->i_lru);
 	address_space_init_once(&inode->i_data);
 	i_size_ordered_init(inode);

fs/super.c +2 -0

index 954aeb8..86822b8 100644
@@ -206,6 +206,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
 	mutex_init(&s->s_sync_lock);
 	INIT_LIST_HEAD(&s->s_inodes);
 	spin_lock_init(&s->s_inode_list_lock);
+	INIT_LIST_HEAD(&s->s_inodes_wb);
+	spin_lock_init(&s->s_inode_wblist_lock);
 
 	if (list_lru_init_memcg(&s->s_dentry_lru))
 		goto fail;

include/linux/fs.h +4 -0

index ef3cd36..aef01c7 100644
@@ -648,6 +648,7 @@ struct inode {
 #endif
 	struct list_head	i_lru;		/* inode LRU list */
 	struct list_head	i_sb_list;
+	struct list_head	i_wb_list;	/* backing dev writeback list */
 	union {
 		struct hlist_head	i_dentry;
 		struct rcu_head		i_rcu;

@@ -1374,6 +1375,9 @@ struct super_block {
 	/* s_inode_list_lock protects s_inodes */
 	spinlock_t		s_inode_list_lock ____cacheline_aligned_in_smp;
 	struct list_head	s_inodes;	/* all inodes */
+
+	spinlock_t		s_inode_wblist_lock;
+	struct list_head	s_inodes_wb;	/* writeback inodes */
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);

include/linux/writeback.h +3 -0

index b333c94..90a380c 100644
@@ -379,4 +379,7 @@ void tag_pages_for_writeback(struct address_space *mapping,
 
 void account_page_redirty(struct page *page);
 
+void sb_mark_inode_writeback(struct inode *inode);
+void sb_clear_inode_writeback(struct inode *inode);
+
 #endif		/* WRITEBACK_H */

mm/page-writeback.c +19 -0

index d15d88c..dcd4e2b 100644
@@ -2735,6 +2735,11 @@ int test_clear_page_writeback(struct page *page)
 				__wb_writeout_inc(wb);
 			}
 		}
+
+		if (mapping->host && !mapping_tagged(mapping,
+						     PAGECACHE_TAG_WRITEBACK))
+			sb_clear_inode_writeback(mapping->host);
+
 		spin_unlock_irqrestore(&mapping->tree_lock, flags);
 	} else {
 		ret = TestClearPageWriteback(page);

@@ -2763,11 +2768,25 @@ int __test_set_page_writeback(struct page *page, bool keep_write)
 		spin_lock_irqsave(&mapping->tree_lock, flags);
 		ret = TestSetPageWriteback(page);
 		if (!ret) {
+			bool on_wblist;
+
+			on_wblist = mapping_tagged(mapping,
+						   PAGECACHE_TAG_WRITEBACK);
+
 			radix_tree_tag_set(&mapping->page_tree,
 						page_index(page),
 						PAGECACHE_TAG_WRITEBACK);
 			if (bdi_cap_account_writeback(bdi))
 				__inc_wb_stat(inode_to_wb(inode), WB_WRITEBACK);
+
+			/*
+			 * we can come through here when swapping anonymous
+			 * pages, so we don't necessarily have an inode to
+			 * track for sync. Inodes are remove lazily, so there is
+			 * no equivalent in test_clear_page_writeback().
+			 */
+			if (!on_wblist && mapping->host)
+				sb_mark_inode_writeback(mapping->host);
 		}
 		if (!PageDirty(page))
 			radix_tree_tag_clear(&mapping->page_tree,

Signature · 6 lines

2.4.3

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From: Dave Chinner <[email protected]>

wait_sb_inodes() currently does a walk of all inodes in the filesystem to find dirty one to wait on during sync. This is highly inefficient and wastes a lot of CPU when there are lots of clean cached inodes that we don't need to wait on.

To avoid this "all inode" walk, we need to track inodes that are currently under writeback that we need to wait for. We do this by adding inodes to a writeback list on the sb when the mapping is first tagged as having pages under writeback. wait_sb_inodes() can then walk this list of "inodes under IO" and wait specifically just for the inodes that the current sync(2) needs to wait for.

Define a couple helpers to add/remove an inode from the writeback list and call them when the overall mapping is tagged for or cleared from writeback. Update wait_sb_inodes() to walk only the inodes under writeback due to the sync.

Signed-off-by Dave Chinner <[email protected]>

Signed-off-by Josef Bacik <[email protected]>

Signed-off-by Brian Foster <[email protected]>

void inode_wait_for_writeback(struct inode *inode) {

fs/fs-writeback.c +2 -0

+	BUG_ON(!(inode->i_state & I_FREEING));
+
 	spin_lock(&inode->i_lock);
 	__inode_wait_for_writeback(inode);
 	spin_unlock(&inode->i_lock);

fs/fs-writeback.c +19 -1

+		sb_clear_inode_writeback(inode);
+	BUG_ON(!list_empty(&inode->i_wb_list));
 }
 
 /*

@@ -2108,7 +2154,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
  */
 static void wait_sb_inodes(struct super_block *sb)
 {
-	struct inode *inode, *old_inode = NULL;
+	LIST_HEAD(sync_list);
 
 	/*
 	 * We need to be protected against the filesystem going from

@@ -2116,23 +2162,56 @@ static void wait_sb_inodes(struct super_block *sb)
 	 */
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback, because
+	 * there may have been pages dirtied before our sync call, but which had
+	 * writeout started before we write it out.  In which case, the inode
+	 * may not be on the dirty list, but we still have to wait for that
+	 * writeout.
+	 *
+	 * To avoid syncing inodes put under IO after we have started here,
+	 * splice the io list to a temporary list head and walk that. Newly
+	 * dirtied inodes will go onto the primary list so we won't wait for
+	 * them. This is safe to do as we are serialised by the s_sync_lock,
+	 * so we'll complete processing the complete list before the next
+	 * sync operation repeats the splice-and-walk process.
+	 *
+	 * s_inode_wblist_lock protects the wb list and is irq-safe as it is
+	 * acquired inside of the mapping lock by __test_set_page_writeback().

fs/fs-writeback.c +2 -4

+	spin_lock_irq(&sb->s_inode_wblist_lock);
+	list_splice_init(&sb->s_inodes_wb, &sync_list);
 
-	/*
-	 * Data integrity sync. Must wait for all pages under writeback,
-	 * because there may have been pages dirtied before our sync
-	 * call, but which had writeout started before we write it out.

fs/fs-writeback.c +23 -1

+		/*
+		 * The inode has been written back. Check whether we still have
+		 * pages under I/O and move the inode back to the primary list
+		 * if necessary. sb_mark_inode_writeback() might not have done
+		 * anything if the writeback tag hadn't been cleared from the
+		 * mapping by the time more wb had started.
+		 */
+		spin_lock_irq(&mapping->tree_lock);
+		if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
+			spin_lock(&sb->s_inode_wblist_lock);
+			if (list_empty(&inode->i_wb_list))
+				list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
+			spin_unlock(&sb->s_inode_wblist_lock);
+		} else
+			WARN_ON(!list_empty(&inode->i_wb_list));
+		spin_unlock_irq(&mapping->tree_lock);
+
+		iput(inode);
+
 		spin_lock(&sb->s_inode_list_lock);
+		spin_lock_irq(&sb->s_inode_wblist_lock);
 	}
+
+	spin_unlock_irq(&sb->s_inode_wblist_lock);
 	spin_unlock(&sb->s_inode_list_lock);
-	iput(old_inode);
+
 	mutex_unlock(&sb->s_sync_lock);
 }

Quoted 22 lines · Brian Foster

From: Dave Chinner <[email protected]>

wait_sb_inodes() currently does a walk of all inodes in the filesystem to find dirty one to wait on during sync. This is highly inefficient and wastes a lot of CPU when there are lots of clean cached inodes that we don't need to wait on.

To avoid this "all inode" walk, we need to track inodes that are currently under writeback that we need to wait for. We do this by adding inodes to a writeback list on the sb when the mapping is first tagged as having pages under writeback. wait_sb_inodes() can then walk this list of "inodes under IO" and wait specifically just for the inodes that the current sync(2) needs to wait for.

Define a couple helpers to add/remove an inode from the writeback list and call them when the overall mapping is tagged for or cleared from writeback. Update wait_sb_inodes() to walk only the inodes under writeback due to the sync.

Signed-off-by Dave Chinner <[email protected]>

Signed-off-by Josef Bacik <[email protected]>

Signed-off-by Brian Foster <[email protected]>

...

Quoted 12 lines

void inode_wait_for_writeback(struct inode *inode) {

fs/fs-writeback.c +7 -0

+	BUG_ON(!(inode->i_state & I_FREEING));
+
 	spin_lock(&inode->i_lock);
 	__inode_wait_for_writeback(inode);
 	spin_unlock(&inode->i_lock);
+
+	/*
+	 * For a bd_inode when we do inode_to_bdi we'll want to get the bdev for
+	 * the inode and then deref bdev->bd_disk, which at this point has been
+	 * set to NULL, so we would panic.  At the point we are dropping our

fs/fs-writeback.c static void __inode_wait_for_writeback(struct inode *inode) · line 1171

Quoted 5 lines

fs/fs-writeback.c +5 -0

+	 * bd_inode we won't have any pages under writeback on the device so
+	 * this is safe.  But just in case we'll assert to make sure we don't
+	 * screw this up.
+	 */
+	if (!sb_is_blkdev_sb(inode->i_sb))

The condition and the comment can go away now. We no longer need to call inode_to_bdi()...

Quoted 41 lines

fs/fs-writeback.c +26 -1

+		sb_clear_inode_writeback(inode);
+	BUG_ON(!list_empty(&inode->i_wb_list));
 }
 
 /*

@@ -2108,7 +2154,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
  */
 static void wait_sb_inodes(struct super_block *sb)
 {
-	struct inode *inode, *old_inode = NULL;
+	LIST_HEAD(sync_list);
 
 	/*
 	 * We need to be protected against the filesystem going from

@@ -2116,23 +2162,56 @@ static void wait_sb_inodes(struct super_block *sb)
 	 */
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback, because
+	 * there may have been pages dirtied before our sync call, but which had
+	 * writeout started before we write it out.  In which case, the inode
+	 * may not be on the dirty list, but we still have to wait for that
+	 * writeout.
+	 *
+	 * To avoid syncing inodes put under IO after we have started here,
+	 * splice the io list to a temporary list head and walk that. Newly
+	 * dirtied inodes will go onto the primary list so we won't wait for
+	 * them. This is safe to do as we are serialised by the s_sync_lock,
+	 * so we'll complete processing the complete list before the next
+	 * sync operation repeats the splice-and-walk process.
+	 *
+	 * s_inode_wblist_lock protects the wb list and is irq-safe as it is
+	 * acquired inside of the mapping lock by __test_set_page_writeback().
+	 * We cannot acquire i_lock while the wblist lock is held without
+	 * introducing irq inversion issues. Since s_inodes_wb is a subset of
+	 * s_inodes, use s_inode_list_lock to prevent inodes from disappearing
+	 * until we have a reference. Note that s_inode_wblist_lock protects the
+	 * local sync_list as well because inodes can be dropped from either
+	 * list by writeback completion.
+	 */

fs/fs-writeback.c static void wait_sb_inodes(struct super_block *sb) · line 2190

Quoted 3 lines

fs/fs-writeback.c +1 -0

 	mutex_lock(&sb->s_sync_lock);
+
 	spin_lock(&sb->s_inode_list_lock);

So I'm not very happy that we have to hold s_inode_list_lock here. After all reducing contention on that lock was one of the main motivations of this patch. That being said I think the locking is correct now which is a good start :) and we can work from here.

As a future improvement I think we could use RCU to grab inode reference safely like:

Hold rcu_read_lock() instead of s_inode_list_lock. That guarantees that memory for the inode we have reference to is not freed. So we can then:

	spin_unlock_irq(&sb->s_inode_wblist_lock);
	spin_lock(&inode->i_lock);
	if (inode->i_flags & (I_FREEING | I_WILL_FREE | I_NEW))
		avoid touching inode
	__iget(inode);
	spin_unlock(&inode->i_lock);
	rcu_read_unlock();

But I'd do this as a separate patch and run this through Al to verify...

Quoted 7 lines

fs/fs-writeback.c +2 -4

+	spin_lock_irq(&sb->s_inode_wblist_lock);
+	list_splice_init(&sb->s_inodes_wb, &sync_list);
 
-	/*
-	 * Data integrity sync. Must wait for all pages under writeback,
-	 * because there may have been pages dirtied before our sync
-	 * call, but which had writeout started before we write it out.

fs/fs-writeback.c static void wait_sb_inodes(struct super_block *sb) · line 2199

Quoted 10 lines

fs/fs-writeback.c +4 -4

-	 * In which case, the inode may not be on the dirty list, but
-	 * we still have to wait for that writeout.
-	 */
-	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+	while (!list_empty(&sync_list)) {
+		struct inode *inode = list_first_entry(&sync_list, struct inode,
+						       i_wb_list);
 		struct address_space *mapping = inode->i_mapping;
 
+		list_del_init(&inode->i_wb_list);

What if we just did:

		list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);

here instead of list_del_init()? That way we would always maintain consistency:

PAGECACHE_TAG_WRITEBACK set iff !list_empty(&inode->i_wb_list)

under s_inode_wblist_lock and thus you could just delete that list shuffling below...

Quoted 30 lines

fs/fs-writeback.c +23 -1

+		/*
+		 * The inode has been written back. Check whether we still have
+		 * pages under I/O and move the inode back to the primary list
+		 * if necessary. sb_mark_inode_writeback() might not have done
+		 * anything if the writeback tag hadn't been cleared from the
+		 * mapping by the time more wb had started.
+		 */
+		spin_lock_irq(&mapping->tree_lock);
+		if (mapping_tagged(mapping, PAGECACHE_TAG_WRITEBACK)) {
+			spin_lock(&sb->s_inode_wblist_lock);
+			if (list_empty(&inode->i_wb_list))
+				list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);
+			spin_unlock(&sb->s_inode_wblist_lock);
+		} else
+			WARN_ON(!list_empty(&inode->i_wb_list));
+		spin_unlock_irq(&mapping->tree_lock);
+
+		iput(inode);
+
 		spin_lock(&sb->s_inode_list_lock);
+		spin_lock_irq(&sb->s_inode_wblist_lock);
 	}
+
+	spin_unlock_irq(&sb->s_inode_wblist_lock);
 	spin_unlock(&sb->s_inode_list_lock);
-	iput(old_inode);
+
 	mutex_unlock(&sb->s_sync_lock);
 }

...

mm/page-writeback.c int __test_set_page_writeback(struct page *page, bool keep_write) · line 2786

Quoted 6 lines

mm/page-writeback.c +6 -0

+
+			/*
+			 * we can come through here when swapping anonymous
+			 * pages, so we don't necessarily have an inode to
+			 * track for sync. Inodes are remove lazily, so there is
+			 * no equivalent in test_clear_page_writeback().

The comment about lazy removal is not true anymore...

Honza

Signature · 2 lines

Jan Kara <[email protected]>
SUSE Labs, CR

Quoted 38 lines · Jan Kara

Quoted 22 lines · Brian Foster

From: Dave Chinner <[email protected]>

wait_sb_inodes() currently does a walk of all inodes in the filesystem to find dirty one to wait on during sync. This is highly inefficient and wastes a lot of CPU when there are lots of clean cached inodes that we don't need to wait on.

To avoid this "all inode" walk, we need to track inodes that are currently under writeback that we need to wait for. We do this by adding inodes to a writeback list on the sb when the mapping is first tagged as having pages under writeback. wait_sb_inodes() can then walk this list of "inodes under IO" and wait specifically just for the inodes that the current sync(2) needs to wait for.

Define a couple helpers to add/remove an inode from the writeback list and call them when the overall mapping is tagged for or cleared from writeback. Update wait_sb_inodes() to walk only the inodes under writeback due to the sync.

Signed-off-by Dave Chinner <[email protected]>

Signed-off-by Josef Bacik <[email protected]>

Signed-off-by Brian Foster <[email protected]>

...

Quoted 12 lines

void inode_wait_for_writeback(struct inode *inode) {

fs/fs-writeback.c +7 -0

+	BUG_ON(!(inode->i_state & I_FREEING));
+
 	spin_lock(&inode->i_lock);
 	__inode_wait_for_writeback(inode);
 	spin_unlock(&inode->i_lock);
+
+	/*
+	 * For a bd_inode when we do inode_to_bdi we'll want to get the bdev for
+	 * the inode and then deref bdev->bd_disk, which at this point has been
+	 * set to NULL, so we would panic.  At the point we are dropping our

Quoted 9 lines

fs/fs-writeback.c static void __inode_wait_for_writeback(struct inode *inode) · line 1171

Quoted 5 lines

fs/fs-writeback.c +5 -0

+	 * bd_inode we won't have any pages under writeback on the device so
+	 * this is safe.  But just in case we'll assert to make sure we don't
+	 * screw this up.
+	 */
+	if (!sb_is_blkdev_sb(inode->i_sb))

The condition and the comment can go away now. We no longer need to call inode_to_bdi()...

Yeah, I noted that in my follow on mail. I think this whole hunk along with the change to kill_bdev() can both actually get dropped. kill_bdev() does truncate_inodes_pages() which eventually calls wait_on_page_writeback() for every page in the mapping. This function clearly already waits for inode writeback.

Unless I'm misunderstanding something, the original changes in both places look like they were just there to ensure that the inode is removed from the wb list when wb completion didn't do so. That is no longer necessary now that wb completion takes care of that. I'll do some more testing of course, but any objections if I drop these hunks?

Alternatively, we could retain only the BUG_ON() bits here to make sure everything works as expected. Thoughts?

Quoted 41 lines

Quoted 41 lines

fs/fs-writeback.c +26 -1

+		sb_clear_inode_writeback(inode);
+	BUG_ON(!list_empty(&inode->i_wb_list));
 }
 
 /*

@@ -2108,7 +2154,7 @@ EXPORT_SYMBOL(__mark_inode_dirty);
  */
 static void wait_sb_inodes(struct super_block *sb)
 {
-	struct inode *inode, *old_inode = NULL;
+	LIST_HEAD(sync_list);
 
 	/*
 	 * We need to be protected against the filesystem going from

@@ -2116,23 +2162,56 @@ static void wait_sb_inodes(struct super_block *sb)
 	 */
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
+	/*
+	 * Data integrity sync. Must wait for all pages under writeback, because
+	 * there may have been pages dirtied before our sync call, but which had
+	 * writeout started before we write it out.  In which case, the inode
+	 * may not be on the dirty list, but we still have to wait for that
+	 * writeout.
+	 *
+	 * To avoid syncing inodes put under IO after we have started here,
+	 * splice the io list to a temporary list head and walk that. Newly
+	 * dirtied inodes will go onto the primary list so we won't wait for
+	 * them. This is safe to do as we are serialised by the s_sync_lock,
+	 * so we'll complete processing the complete list before the next
+	 * sync operation repeats the splice-and-walk process.
+	 *
+	 * s_inode_wblist_lock protects the wb list and is irq-safe as it is
+	 * acquired inside of the mapping lock by __test_set_page_writeback().
+	 * We cannot acquire i_lock while the wblist lock is held without
+	 * introducing irq inversion issues. Since s_inodes_wb is a subset of
+	 * s_inodes, use s_inode_list_lock to prevent inodes from disappearing
+	 * until we have a reference. Note that s_inode_wblist_lock protects the
+	 * local sync_list as well because inodes can be dropped from either
+	 * list by writeback completion.
+	 */

Quoted 9 lines

fs/fs-writeback.c static void wait_sb_inodes(struct super_block *sb) · line 2190

Quoted 3 lines

fs/fs-writeback.c +1 -0

 	mutex_lock(&sb->s_sync_lock);
+
 	spin_lock(&sb->s_inode_list_lock);

So I'm not very happy that we have to hold s_inode_list_lock here. After all reducing contention on that lock was one of the main motivations of this patch. That being said I think the locking is correct now which is a good start :) and we can work from here.

Thanks.. I was shooting for correct and as simple as possible. ;) FWIW, the primary motivation that I'm aware of for this patch is the current behavior of unnecessarily walking a massive inode list when the inode cache is very large (moreso than lock contention). I suspect lock contention in this path would more likely involve the wblist_lock at this point, since it protects the wblist.

Quoted 25 lines

Quoted 7 lines

fs/fs-writeback.c +2 -4

+	spin_lock_irq(&sb->s_inode_wblist_lock);
+	list_splice_init(&sb->s_inodes_wb, &sync_list);
 
-	/*
-	 * Data integrity sync. Must wait for all pages under writeback,
-	 * because there may have been pages dirtied before our sync
-	 * call, but which had writeout started before we write it out.

fs/fs-writeback.c static void wait_sb_inodes(struct super_block *sb) · line 2199

Quoted 10 lines

fs/fs-writeback.c +4 -4

-	 * In which case, the inode may not be on the dirty list, but
-	 * we still have to wait for that writeout.
-	 */
-	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
+	while (!list_empty(&sync_list)) {
+		struct inode *inode = list_first_entry(&sync_list, struct inode,
+						       i_wb_list);
 		struct address_space *mapping = inode->i_mapping;
 
+		list_del_init(&inode->i_wb_list);

What if we just did:

		list_add_tail(&inode->i_wb_list, &sb->s_inodes_wb);

here instead of list_del_init()? That way we would always maintain consistency:

Quoted 5 lines

PAGECACHE_TAG_WRITEBACK set iff !list_empty(&inode->i_wb_list)

under s_inode_wblist_lock and thus you could just delete that list shuffling below...

I assume you mean list_move_tail(), but yeah I think you're right. We

can still use the mapping tag check optimization without removing the inode, since there's no risk of leaving it on a local list after the function returns. Beyond that, once the lock is dropped the list state is simply dependent on whether more wb starts after the immediately previous wait. I'll stare at it some more, but it sounds like a nice cleanup to me.