[PATCH v2] btrfs: shrink delalloc pages instead of full inodes

[Josef Bacik <josef@toxicpanda.com>]

Commit 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in
shrink_delalloc") cleaned up how we do delalloc shrinking by utilizing
some infrastructure we have in place to flush inodes that we use for
device replace and snapshot.  However this introduced a pretty serious
performance regression.  To reproduce the user untarred the source
tarball of Firefox, and would see it take anywhere from 5 to 20 times as
long to untar in 5.10 compared to 5.9.

The root cause is because before we would generally use the normal
writeback path to reclaim delalloc space, and for this we would provide
it with the number of pages we wanted to flush.  The referenced commit
changed this to flush that many inodes, which drastically increased the
amount of space we were flushing in certain cases, which severely
affected performance.

We cannot revert this patch unfortunately, because Filipe has another
fix that requires the ability to skip flushing inodes that are being
cloned in certain scenarios, which means we need to keep using our
flushing infrastructure or risk re-introducing the deadlock.

Instead to fix this problem we can go back to providing
btrfs_start_delalloc_roots with a number of pages to flush, and then set
up a writeback_control and utilize sync_inode() to handle the flushing
for us.  This gives us the same behavior we had prior to the fix, while
still allowing us to avoid the deadlock that was fixed by Filipe.  I
redid the users original test and got the following results

5.9	0m54.258s
5.10	1m26.212s
Patched	0m38.800s

We are significantly faster because of the work I did around improving
ENOSPC flushing in 5.10 and 5.11, so reverting to the previous write out
behavior gave us a pretty big boost.

CC: stable@vger.kernel.org # 5.10
Reported-by: René Rebe <rene@exactcode.de>
Fixes: 38d715f494f2 ("btrfs: use btrfs_start_delalloc_roots in shrink_delalloc")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
v1->v2:
- Explicitly state what the regression was in the commit message.

 fs/btrfs/inode.c      | 60 +++++++++++++++++++++++++++++++------------
 fs/btrfs/space-info.c |  4 ++-
 2 files changed, 46 insertions(+), 18 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 070716650df8..a8e0a6b038d3 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -9390,7 +9390,8 @@ 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, u64 *nr, bool snapshot,
+static int start_delalloc_inodes(struct btrfs_root *root,
+				 struct writeback_control *wbc, bool snapshot,
 				 bool in_reclaim_context)
 {
 	struct btrfs_inode *binode;
@@ -9399,6 +9400,7 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot
 	struct list_head works;
 	struct list_head splice;
 	int ret = 0;
+	bool full_flush = wbc->nr_to_write == LONG_MAX;
 
 	INIT_LIST_HEAD(&works);
 	INIT_LIST_HEAD(&splice);
@@ -9427,18 +9429,24 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot
 		if (snapshot)
 			set_bit(BTRFS_INODE_SNAPSHOT_FLUSH,
 				&binode->runtime_flags);
-		work = btrfs_alloc_delalloc_work(inode);
-		if (!work) {
-			iput(inode);
-			ret = -ENOMEM;
-			goto out;
-		}
-		list_add_tail(&work->list, &works);
-		btrfs_queue_work(root->fs_info->flush_workers,
-				 &work->work);
-		if (*nr != U64_MAX) {
-			(*nr)--;
-			if (*nr == 0)
+		if (full_flush) {
+			work = btrfs_alloc_delalloc_work(inode);
+			if (!work) {
+				iput(inode);
+				ret = -ENOMEM;
+				goto out;
+			}
+			list_add_tail(&work->list, &works);
+			btrfs_queue_work(root->fs_info->flush_workers,
+					 &work->work);
+		} else {
+			ret = sync_inode(inode, wbc);
+			if (!ret &&
+			    test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT,
+				     &BTRFS_I(inode)->runtime_flags))
+				ret = sync_inode(inode, wbc);
+			btrfs_add_delayed_iput(inode);
+			if (ret || wbc->nr_to_write <= 0)
 				goto out;
 		}
 		cond_resched();
@@ -9464,18 +9472,29 @@ static int start_delalloc_inodes(struct btrfs_root *root, u64 *nr, bool snapshot
 
 int btrfs_start_delalloc_snapshot(struct btrfs_root *root)
 {
+	struct writeback_control wbc = {
+		.nr_to_write = LONG_MAX,
+		.sync_mode = WB_SYNC_NONE,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+	};
 	struct btrfs_fs_info *fs_info = root->fs_info;
-	u64 nr = U64_MAX;
 
 	if (test_bit(BTRFS_FS_STATE_ERROR, &fs_info->fs_state))
 		return -EROFS;
 
-	return start_delalloc_inodes(root, &nr, true, false);
+	return start_delalloc_inodes(root, &wbc, true, false);
 }
 
 int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr,
 			       bool in_reclaim_context)
 {
+	struct writeback_control wbc = {
+		.nr_to_write = (nr == U64_MAX) ? LONG_MAX : (unsigned long)nr,
+		.sync_mode = WB_SYNC_NONE,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+	};
 	struct btrfs_root *root;
 	struct list_head splice;
 	int ret;
@@ -9489,6 +9508,13 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr,
 	spin_lock(&fs_info->delalloc_root_lock);
 	list_splice_init(&fs_info->delalloc_roots, &splice);
 	while (!list_empty(&splice) && nr) {
+		/*
+		 * Reset nr_to_write here so we know that we're doing a full
+		 * flush.
+		 */
+		if (nr == U64_MAX)
+			wbc.nr_to_write = LONG_MAX;
+
 		root = list_first_entry(&splice, struct btrfs_root,
 					delalloc_root);
 		root = btrfs_grab_root(root);
@@ -9497,9 +9523,9 @@ int btrfs_start_delalloc_roots(struct btrfs_fs_info *fs_info, u64 nr,
 			       &fs_info->delalloc_roots);
 		spin_unlock(&fs_info->delalloc_root_lock);
 
-		ret = start_delalloc_inodes(root, &nr, false, in_reclaim_context);
+		ret = start_delalloc_inodes(root, &wbc, false, in_reclaim_context);
 		btrfs_put_root(root);
-		if (ret < 0)
+		if (ret < 0 || wbc.nr_to_write <= 0)
 			goto out;
 		spin_lock(&fs_info->delalloc_root_lock);
 	}
diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index 67e55c5479b8..e8347461c8dd 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -532,7 +532,9 @@ static void shrink_delalloc(struct btrfs_fs_info *fs_info,
 
 	loops = 0;
 	while ((delalloc_bytes || dio_bytes) && loops < 3) {
-		btrfs_start_delalloc_roots(fs_info, items, true);
+		u64 nr_pages = min(delalloc_bytes, to_reclaim) >> PAGE_SHIFT;
+
+		btrfs_start_delalloc_roots(fs_info, nr_pages, true);
 
 		loops++;
 		if (wait_ordered && !trans) {
-- 
2.26.2