Re: [PATCH v15 40/42] btrfs: zoned: serialize log transaction on zoned filesystems

On Thu, Feb 04, 2021 at 11:50:45AM +0000, Filipe Manana wrote:

On Thu, Feb 4, 2021 at 10:23 AM Naohiro Aota [off-list ref] wrote:

quoted

This is the 2/3 patch to enable tree-log on zoned filesystems.

Since we can start more than one log transactions per subvolume
simultaneously, nodes from multiple transactions can be allocated
interleaved. Such mixed allocation results in non-sequential writes at the
time of a log transaction commit. The nodes of the global log root tree
(fs_info->log_root_tree), also have the same problem with mixed
allocation.

Serializes log transactions by waiting for a committing transaction when
someone tries to start a new transaction, to avoid the mixed allocation
problem. We must also wait for running log transactions from another
subvolume, but there is no easy way to detect which subvolume root is
running a log transaction. So, this patch forbids starting a new log
transaction when other subvolumes already allocated the global log root
tree.

Cc: Filipe Manana <redacted>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
---
 fs/btrfs/tree-log.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index c02eeeac439c..8be3164d4c5d 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c

@@ -105,6 +105,7 @@ static noinline int replay_dir_deletes(struct btrfs_trans_handle *trans,
                                       struct btrfs_root *log,
                                       struct btrfs_path *path,
                                       u64 dirid, int del_all);
+static void wait_log_commit(struct btrfs_root *root, int transid);

 /*
  * tree logging is a special write ahead log used to make sure that

@@ -140,6 +141,7 @@ static int start_log_trans(struct btrfs_trans_handle *trans,
 {
        struct btrfs_fs_info *fs_info = root->fs_info;
        struct btrfs_root *tree_root = fs_info->tree_root;
+       const bool zoned = btrfs_is_zoned(fs_info);
        int ret = 0;

        /*

@@ -160,12 +162,20 @@ static int start_log_trans(struct btrfs_trans_handle *trans,

        mutex_lock(&root->log_mutex);

+again:
        if (root->log_root) {
+               int index = (root->log_transid + 1) % 2;
+
                if (btrfs_need_log_full_commit(trans)) {
                        ret = -EAGAIN;
                        goto out;
                }

+               if (zoned && atomic_read(&root->log_commit[index])) {
+                       wait_log_commit(root, root->log_transid - 1);
+                       goto again;
+               }
+
                if (!root->log_start_pid) {
                        clear_bit(BTRFS_ROOT_MULTI_LOG_TASKS, &root->state);
                        root->log_start_pid = current->pid;

@@ -173,6 +183,17 @@ static int start_log_trans(struct btrfs_trans_handle *trans,
                        set_bit(BTRFS_ROOT_MULTI_LOG_TASKS, &root->state);
                }
        } else {
+               if (zoned) {
+                       mutex_lock(&fs_info->tree_log_mutex);
+                       if (fs_info->log_root_tree)
+                               ret = -EAGAIN;
+                       else
+                               ret = btrfs_init_log_root_tree(trans, fs_info);
+                       mutex_unlock(&fs_info->tree_log_mutex);
+               }

So, nothing here changed since v14 - all my comments still apply [1]
This is based on pre-5.10 code and is broken as it is - it results in
every fsync falling back to a transaction commit, defeating the
purpose of all the patches that deal with log trees on zoned
filesystems.

Thanks.

[1] https://lore.kernel.org/linux-btrfs/CAL3q7H5pv416FVwThOHe+M3L5B-z_n6_ZGQQxsUq5vC5fsAoJw@mail.gmail.com/ (local)

Yes...

As noted in the cover letter, there is a fix for this issue
itself. However, the fix revealed other failures in fsync() path.
But, with further investigation, I found the failures are not really
related to zoned fsync() code. So, I will soon post two patches (one
incremental for this one, and one to deal with a regression case)..

quoted

+               if (ret)
+                       goto out;
+
                ret = btrfs_add_log_tree(trans, root);
                if (ret)
                        goto out;

@@ -201,14 +222,22 @@ static int start_log_trans(struct btrfs_trans_handle *trans,
  */
 static int join_running_log_trans(struct btrfs_root *root)
 {
+       const bool zoned = btrfs_is_zoned(root->fs_info);
        int ret = -ENOENT;

        if (!test_bit(BTRFS_ROOT_HAS_LOG_TREE, &root->state))
                return ret;

        mutex_lock(&root->log_mutex);
+again:
        if (root->log_root) {
+               int index = (root->log_transid + 1) % 2;
+
                ret = 0;
+               if (zoned && atomic_read(&root->log_commit[index])) {
+                       wait_log_commit(root, root->log_transid - 1);
+                       goto again;
+               }
                atomic_inc(&root->log_writers);
        }
        mutex_unlock(&root->log_mutex);
--

2.30.0

-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”