[PATCH] btrfs: use tagged writepage to mitigate livelock of snapshot

[Ethan Lien <ethanlien@synology.com>]

Snapshot is expected to be fast. But if there are writers steadily
create dirty pages in our subvolume, the snapshot may take a very long
time to complete. To fix the problem, we use tagged writepage for snapshot
flusher as we do in the generic write_cache_pages(), so we can ommit pages
dirtied after the snapshot command.

We do a simple snapshot speed test on a Intel D-1531 box:

fio --ioengine=libaio --iodepth=32 --bs=4k --rw=write --size=64G
--direct=0 --thread=1 --numjobs=1 --time_based --runtime=120
--filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5;
time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio

original: 1m58sec
patched:  6.54sec

This is the best case for this patch since for a sequential write case,
we omit nearly all pages dirtied after the snapshot command.

For a multi writers, random write test:

fio --ioengine=libaio --iodepth=32 --bs=4k --rw=randwrite --size=64G
--direct=0 --thread=1 --numjobs=4 --time_based --runtime=120
--filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5;
time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio

original: 15.83sec
patched:  10.35sec

The improvement is less compared with the sequential write case, since
we omit only half of the pages dirtied after snapshot command.

Signed-off-by: Ethan Lien <ethanlien@synology.com>
---
 fs/btrfs/btrfs_inode.h |  1 +
 fs/btrfs/ctree.h       |  2 +-
 fs/btrfs/extent_io.c   | 16 ++++++++++++++--
 fs/btrfs/inode.c       | 10 ++++++----
 fs/btrfs/ioctl.c       |  2 +-
 5 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 1343ac57b438..4182bfbb56be 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -29,6 +29,7 @@ enum {
 	BTRFS_INODE_IN_DELALLOC_LIST,
 	BTRFS_INODE_READDIO_NEED_LOCK,
 	BTRFS_INODE_HAS_PROPS,
+	BTRFS_INODE_TAGGED_FLUSH,
 };
 
 /* in memory btrfs inode */
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 2cddfe7806a4..82682da5a40d 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -3155,7 +3155,7 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 			       struct inode *inode, u64 new_size,
 			       u32 min_type);
 
-int btrfs_start_delalloc_inodes(struct btrfs_root *root);
+int btrfs_start_delalloc_snapshot(struct btrfs_root *root);
 int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, int nr);
 int btrfs_set_extent_delalloc(struct inode *inode, u64 start, u64 end,
 			      unsigned int extra_bits,
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 4dd6faab02bb..c21d8a0e010a 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3928,12 +3928,24 @@ static int extent_write_cache_pages(struct address_space *mapping,
 			range_whole = 1;
 		scanned = 1;
 	}
-	if (wbc->sync_mode == WB_SYNC_ALL)
+
+	/*
+	 * We don't care if we are the one who set BTRFS_INODE_TAGGED_FLUSH in
+	 * start_delalloc_inodes(). We do the tagged writepage as long as we are
+	 * the first one who do the filemap_flush() on this inode.
+	 */
+	if (range_whole && wbc->nr_to_write == LONG_MAX &&
+			wbc->sync_mode == WB_SYNC_NONE &&
+			test_and_clear_bit(BTRFS_INODE_TAGGED_FLUSH,
+				&BTRFS_I(inode)->runtime_flags))
+		wbc->tagged_writepages = 1;
+
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
 		tag = PAGECACHE_TAG_TOWRITE;
 	else
 		tag = PAGECACHE_TAG_DIRTY;
 retry:
-	if (wbc->sync_mode == WB_SYNC_ALL)
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
 		tag_pages_for_writeback(mapping, index, end);
 	done_index = index;
 	while (!done && !nr_to_write_done && (index <= end) &&
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 3ea5339603cf..3df3cbbe91c5 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -9975,7 +9975,7 @@ static struct btrfs_delalloc_work *btrfs_alloc_delalloc_work(struct inode *inode
  * some fairly slow code that needs optimization. This walks the list
  * of all the inodes with pending delalloc and forces them to disk.
  */
-static int start_delalloc_inodes(struct btrfs_root *root, int nr)
+static int start_delalloc_inodes(struct btrfs_root *root, int nr, int snapshot)
 {
 	struct btrfs_inode *binode;
 	struct inode *inode;
@@ -10003,6 +10003,8 @@ static int start_delalloc_inodes(struct btrfs_root *root, int nr)
 		}
 		spin_unlock(&root->delalloc_lock);
 
+		if (snapshot)
+			set_bit(BTRFS_INODE_TAGGED_FLUSH, &binode->runtime_flags);
 		work = btrfs_alloc_delalloc_work(inode);
 		if (!work) {
 			iput(inode);
@@ -10036,7 +10038,7 @@ static int start_delalloc_inodes(struct btrfs_root *root, int nr)
 	return ret;
 }
 
-int btrfs_start_delalloc_inodes(struct btrfs_root *root)
+int btrfs_start_delalloc_snapshot(struct btrfs_root *root)
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
 	int ret;
@@ -10044,7 +10046,7 @@ int btrfs_start_delalloc_inodes(struct btrfs_root *root)
 	if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
 		return -EROFS;
 
-	ret = start_delalloc_inodes(root, -1);
+	ret = start_delalloc_inodes(root, -1, 1);
 	if (ret > 0)
 		ret = 0;
 	return ret;
@@ -10073,7 +10075,7 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, int nr)
 			       &fs_info->delalloc_roots);
 		spin_unlock(&fs_info->delalloc_root_lock);
 
-		ret = start_delalloc_inodes(root, nr);
+		ret = start_delalloc_inodes(root, nr, 0);
 		btrfs_put_fs_root(root);
 		if (ret < 0)
 			goto out;
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index d60b6caf09e8..d1293b6c31f6 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -775,7 +775,7 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	wait_event(root->subv_writers->wait,
 		   percpu_counter_sum(&root->subv_writers->counter) == 0);
 
-	ret = btrfs_start_delalloc_inodes(root);
+	ret = btrfs_start_delalloc_snapshot(root);
 	if (ret)
 		goto dec_and_free;
 
-- 
2.19.1