xfs: launder in-memory btree buffers before transaction commit

As we've noted in various places, all current users of in-memory btrees
are online fsck.  Online fsck only stages a btree long enough to rebuild
an ondisk data structure, which means that the in-memory btree is
ephemeral.  Furthermore, if we encounter /any/ errors while updating an
in-memory btree, all we do is tear down all the staged data and return
an errno to userspace.  In-memory btrees need not be transactional, so
their buffers should not be committed to the ondisk log, nor should they
be checkpointed by the AIL.  That's just as well since the ephemeral
nature of the btree means that the buftarg and the buffers may disappear
quickly anyway.

Therefore, we need a way to launder the btree buffers that get attached
to the transaction by the generic btree code.  Because the buffers are
directly mapped to backing file pages, there's no need to bwrite them
back to the tmpfs file.  All we need to do is clean enough of the buffer
log item state so that the bli can be detached from the buffer, remove
the bli from the transaction's log item list, and reset the transaction
dirty state as if the laundered items had never been there.

For simplicity, create xfbtree transaction commit and cancel helpers
that launder the in-memory btree buffers for callers.  Once laundered,
call the write verifier on non-stale buffers to avoid integrity issues,
or punch a hole in the backing file for stale buffers.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 42 insertions, 0 deletions

diff --git a/fs/xfs/xfs_trans_buf.c b/fs/xfs/xfs_trans_buf.c
index 6549e50d852c..e28ab74af4f0 100644
--- a/fs/xfs/xfs_trans_buf.c
+++ b/fs/xfs/xfs_trans_buf.c
@@ -393,6 +393,48 @@ xfs_trans_brelse(
 }
 
 /*
+ * Forcibly detach a buffer previously joined to the transaction.  The caller
+ * will retain its locked reference to the buffer after this function returns.
+ * The buffer must be completely clean and must not be held to the transaction.
+ */
+void
+xfs_trans_bdetach(
+	struct xfs_trans	*tp,
+	struct xfs_buf		*bp)
+{
+	struct xfs_buf_log_item	*bip = bp->b_log_item;
+
+	ASSERT(tp != NULL);
+	ASSERT(bp->b_transp == tp);
+	ASSERT(bip->bli_item.li_type == XFS_LI_BUF);
+	ASSERT(atomic_read(&bip->bli_refcount) > 0);
+
+	trace_xfs_trans_bdetach(bip);
+
+	/*
+	 * Erase all recursion count, since we're removing this buffer from the
+	 * transaction.
+	 */
+	bip->bli_recur = 0;
+
+	/*
+	 * The buffer must be completely clean.  Specifically, it had better
+	 * not be dirty, stale, logged, ordered, or held to the transaction.
+	 */
+	ASSERT(!test_bit(XFS_LI_DIRTY, &bip->bli_item.li_flags));
+	ASSERT(!(bip->bli_flags & XFS_BLI_DIRTY));
+	ASSERT(!(bip->bli_flags & XFS_BLI_HOLD));
+	ASSERT(!(bip->bli_flags & XFS_BLI_LOGGED));
+	ASSERT(!(bip->bli_flags & XFS_BLI_ORDERED));
+	ASSERT(!(bip->bli_flags & XFS_BLI_STALE));
+
+	/* Unlink the log item from the transaction and drop the log item. */
+	xfs_trans_del_item(&bip->bli_item);
+	xfs_buf_item_put(bip);
+	bp->b_transp = NULL;
+}
+
+/*
  * Mark the buffer as not needing to be unlocked when the buf item's
  * iop_committing() routine is called.  The buffer must already be locked
  * and associated with the given transaction.