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

[Naohiro Aota <naohiro.aota@wdc.com>]

On Thu, Feb 04, 2021 at 11:50:45AM +0000, Filipe Manana wrote:
> On Thu, Feb 4, 2021 at 10:23 AM Naohiro Aota <naohiro.aota@wdc.com> wrote:
> >
> > 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 <fdmanana@gmail.com>
> > 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/

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)..

> 
> 
> > +               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.”