Re: [PATCH v15 43/43] btrfs: zoned: deal with holes writing out tree-log pages

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

On Fri, Feb 05, 2021 at 02:19:50PM +0000, Filipe Manana wrote:
> On Fri, Feb 5, 2021 at 11:49 AM Filipe Manana <fdmanana@gmail.com> wrote:
> >
> > On Fri, Feb 5, 2021 at 9:26 AM Naohiro Aota <naohiro.aota@wdc.com> wrote:
> > >
> > > Since the zoned filesystem requires sequential write out of metadata, we
> > > cannot proceed with a hole in tree-log pages. When such a hole exists,
> > > btree_write_cache_pages() will return -EAGAIN. We cannot wait for the range
> > > to be written, because it will cause a deadlock. So, let's bail out to a
> > > full commit in this case.
> > >
> > > Cc: Filipe Manana <fdmanana@gmail.com>
> > > Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> > > ---
> > >  fs/btrfs/tree-log.c | 19 ++++++++++++++++++-
> > >  1 file changed, 18 insertions(+), 1 deletion(-)
> > >
> > > This patch solves a regression introduced by fixing patch 40. I'm
> > > sorry for the confusing patch numbering.
> >
> > Hum, how does patch 40 can cause this?
> > And is it before the fixup or after?
> >
> > >
> > > diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> > > index 4e72794342c0..629e605cd62d 100644
> > > --- a/fs/btrfs/tree-log.c
> > > +++ b/fs/btrfs/tree-log.c
> > > @@ -3120,6 +3120,14 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
> > >          */
> > >         blk_start_plug(&plug);
> > >         ret = btrfs_write_marked_extents(fs_info, &log->dirty_log_pages, mark);
> > > +       /*
> > > +        * There is a hole writing out the extents and cannot proceed it on
> > > +        * zoned filesystem, which require sequential writing. We can
> >
> > require -> requires
> >
> > > +        * ignore the error for now, since we don't wait for completion for
> > > +        * now.
> >
> > So why can we ignore the error for now?
> > Why not just bail out here and mark the log for full commit? (without
> > a transaction abort)
> >
> > > +        */
> > > +       if (ret == -EAGAIN)
> > > +               ret = 0;
> 
> Thinking again about this, it would be safer, and self-documenting to
> check here that we are in zoned mode:
> 
> if (ret == -EAGAIN && is_zoned)
>     ret = 0;
> 
> Because if we start to get -EAGAIN here one day, from non-zoned code,
> we risk not writing out some extent buffer and getting a corrupt log,
> which may be very hard to find.
> With that additional check in place, we'll end up aborting the
> transaction with -EAGAIN and notice the problem much sooner.

Yeah, I agree.

I'll post a new version with the comments revised and using "if (ret
== -EAGAIN && btrfs_is_zoned(fs_info))".

> > >         if (ret) {
> > >                 blk_finish_plug(&plug);
> > >                 btrfs_abort_transaction(trans, ret);
> > > @@ -3229,7 +3237,16 @@ int btrfs_sync_log(struct btrfs_trans_handle *trans,
> > >                                          &log_root_tree->dirty_log_pages,
> > >                                          EXTENT_DIRTY | EXTENT_NEW);
> > >         blk_finish_plug(&plug);
> > > -       if (ret) {
> > > +       /*
> > > +        * There is a hole in the extents, and failed to sequential write
> > > +        * on zoned filesystem. We cannot wait for this write outs, sinc it
> >
> > this -> these
> >
> > > +        * cause a deadlock. Bail out to the full commit, instead.
> > > +        */
> > > +       if (ret == -EAGAIN) {
> 
> I would add "&& is_zoned" here too.
> 
> Thanks.
>
> 
> > > +               btrfs_wait_tree_log_extents(log, mark);
> > > +               mutex_unlock(&log_root_tree->log_mutex);
> > > +               goto out_wake_log_root;
> >
> > Must also call btrfs_set_log_full_commit(trans);
> >
> > Thanks.
> >
> > > +       } else if (ret) {
> > >                 btrfs_set_log_full_commit(trans);
> > >                 btrfs_abort_transaction(trans, ret);
> > >                 mutex_unlock(&log_root_tree->log_mutex);
> > > --
> > > 2.30.0
> > >
> >
> >
> > --
> > Filipe David Manana,
> >
> > “Whether you think you can, or you think you can't — you're right.”
> 
> 
> 
> --
> Filipe David Manana,
> 
> “Whether you think you can, or you think you can't — you're right.”