linux-kernel - Re: [PATCH 02/21] bcachefs: Accumulate accounting keys in journal replay

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Date: Wed, 28 Feb 2024 15:06:02 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Brian Foster <bfoster@...hat.com>
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org, 
	djwong@...nel.org
Subject: Re: [PATCH 02/21] bcachefs: Accumulate accounting keys in journal
 replay

On Tue, Feb 27, 2024 at 10:49:46AM -0500, Brian Foster wrote:
> On Sat, Feb 24, 2024 at 09:38:04PM -0500, Kent Overstreet wrote:
> > Until accounting keys hit the btree, they are deltas, not new versions
> > of the existing key; this means we have to teach journal replay to
> > accumulate them.
> > 
> > Additionally, the journal doesn't track precisely which entries have
> > been flushed to the btree; it only tracks a range of entries that may
> > possibly still need to be flushed.
> > 
> > That means we need to compare accounting keys against the version in the
> > btree and only flush updates that are newer.
> > 
> > There's another wrinkle with the write buffer: if the write buffer
> > starts flushing accounting keys before journal replay has finished
> > flushing accounting keys, journal replay will see the version number
> > from the new updates and updates from the journal will be lost.
> > 
> > To avoid this, journal replay has to flush accounting keys first, and
> > we'll be adding a flag so that write buffer flush knows to hold
> > accounting keys until then.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > ---
> >  fs/bcachefs/btree_journal_iter.c | 23 +++-------
> >  fs/bcachefs/btree_journal_iter.h | 15 +++++++
> >  fs/bcachefs/btree_trans_commit.c |  9 +++-
> >  fs/bcachefs/btree_update.h       | 14 +++++-
> >  fs/bcachefs/recovery.c           | 76 +++++++++++++++++++++++++++++++-
> >  5 files changed, 117 insertions(+), 20 deletions(-)
> > 
> ...
> > diff --git a/fs/bcachefs/recovery.c b/fs/bcachefs/recovery.c
> > index 96e7a1ec7091..6829d80bd181 100644
> > --- a/fs/bcachefs/recovery.c
> > +++ b/fs/bcachefs/recovery.c
> > @@ -11,6 +11,7 @@
> >  #include "btree_io.h"
> >  #include "buckets.h"
> >  #include "dirent.h"
> > +#include "disk_accounting.h"
> >  #include "ec.h"
> >  #include "errcode.h"
> >  #include "error.h"
> > @@ -87,6 +88,56 @@ static void replay_now_at(struct journal *j, u64 seq)
> >  		bch2_journal_pin_put(j, j->replay_journal_seq++);
> >  }
> >  
> > +static int bch2_journal_replay_accounting_key(struct btree_trans *trans,
> > +					      struct journal_key *k)
> > +{
> > +	struct journal_keys *keys = &trans->c->journal_keys;
> > +
> > +	struct btree_iter iter;
> > +	bch2_trans_node_iter_init(trans, &iter, k->btree_id, k->k->k.p,
> > +				  BTREE_MAX_DEPTH, k->level,
> > +				  BTREE_ITER_INTENT);
> > +	int ret = bch2_btree_iter_traverse(&iter);
> > +	if (ret)
> > +		goto out;
> > +
> > +	struct bkey u;
> > +	struct bkey_s_c old = bch2_btree_path_peek_slot(btree_iter_path(trans, &iter), &u);
> > +
> > +	if (bversion_cmp(old.k->version, k->k->k.version) >= 0) {
> > +		ret = 0;
> > +		goto out;
> > +	}
> 
> So I assume this is what correlates back to the need to not flush the
> write buffer until replay completes, otherwise we could unintentionally
> skip subsequent key updates. Is that the case?

No, this is the "has this delta been applie to the btree key" check -
adding that as a comment.

Write buffer exclusion comes with a new filesytem bit that gets set once
accounting keys have all been replayed, that's in the next patch