* - For each work item attached to the log intent item,
* * Perform the described action.
* * Attach the work item to the log done item.
+ * * If the result of doing the work was -EAGAIN, ->finish work
+ * wants a new transaction. See the "Requesting a Fresh
+ * Transaction while Finishing Deferred Work" section below for
+ * details.
*
* The key here is that we must log an intent item for all pending
* work items every time we roll the transaction, and that we must log
* we can perform complex remapping operations, chaining intent items
* as needed.
*
+ * Requesting a Fresh Transaction while Finishing Deferred Work
+ *
+ * If ->finish_item decides that it needs a fresh transaction to
+ * finish the work, it must ask its caller (xfs_defer_finish) for a
+ * continuation. The most likely cause of this circumstance are the
+ * refcount adjust functions deciding that they've logged enough items
+ * to be at risk of exceeding the transaction reservation.
+ *
+ * To get a fresh transaction, we want to log the existing log done
+ * item to prevent the log intent item from replaying, immediately log
+ * a new log intent item with the unfinished work items, roll the
+ * transaction, and re-call ->finish_item wherever it left off. The
+ * log done item and the new log intent item must be in the same
+ * transaction or atomicity cannot be guaranteed; defer_finish ensures
+ * that this happens.
+ *
+ * This requires some coordination between ->finish_item and
+ * defer_finish. Upon deciding to request a new transaction,
+ * ->finish_item should update the current work item to reflect the
+ * unfinished work. Next, it should reset the log done item's list
+ * count to the number of items finished, and return -EAGAIN.
+ * defer_finish sees the -EAGAIN, logs the new log intent item
+ * with the remaining work items, and leaves the xfs_defer_pending
+ * item at the head of the dop_work queue. Then it rolls the
+ * transaction and picks up processing where it left off. It is
+ * required that ->finish_item must be careful to leave enough
+ * transaction reservation to fit the new log intent item.
+ *
* This is an example of remapping the extent (E, E+B) into file X at
* offset A and dealing with the extent (C, C+B) already being mapped
* there:
* | Intent to add rmap (X, E, A, B) |
* +-------------------------------------------------+
* | Reduce refcount for extent (C, B) | t2
- * | Done reducing refcount for extent (C, B) |
+ * | Done reducing refcount for extent (C, 9) |
+ * | Intent to reduce refcount for extent (C+9, B-9) |
+ * | (ran out of space after 9 refcount updates) |
+ * +-------------------------------------------------+
+ * | Reduce refcount for extent (C+9, B+9) | t3
+ * | Done reducing refcount for extent (C+9, B-9) |
* | Increase refcount for extent (E, B) |
* | Done increasing refcount for extent (E, B) |
* | Intent to free extent (C, B) |
* | Intent to free extent (F, 1) (refcountbt block) |
* | Intent to remove rmap (F, 1, REFC) |
* +-------------------------------------------------+
- * | Remove rmap (X, C, A, B) | t3
+ * | Remove rmap (X, C, A, B) | t4
* | Done removing rmap (X, C, A, B) |
* | Add rmap (X, E, A, B) |
* | Done adding rmap (X, E, A, B) |
* | Remove rmap (F, 1, REFC) |
* | Done removing rmap (F, 1, REFC) |
* +-------------------------------------------------+
- * | Free extent (C, B) | t4
+ * | Free extent (C, B) | t5
* | Done freeing extent (C, B) |
* | Free extent (D, 1) |
* | Done freeing extent (D, 1) |
* - Intent to free extent (C, B)
* - Intent to free extent (F, 1) (refcountbt block)
* - Intent to remove rmap (F, 1, REFC)
+ *
+ * Note that the continuation requested between t2 and t3 is likely to
+ * reoccur.
*/
static const struct xfs_defer_op_type *defer_op_types[XFS_DEFER_OPS_TYPE_MAX];
dfp->dfp_count--;
error = dfp->dfp_type->finish_item(*tp, dop, li,
dfp->dfp_done, &state);
- if (error) {
+ if (error == -EAGAIN) {
+ /*
+ * Caller wants a fresh transaction;
+ * put the work item back on the list
+ * and jump out.
+ */
+ list_add(li, &dfp->dfp_work);
+ dfp->dfp_count++;
+ break;
+ } else if (error) {
/*
* Clean up after ourselves and jump out.
* xfs_defer_cancel will take care of freeing
goto out;
}
}
- /* Done with the dfp, free it. */
- list_del(&dfp->dfp_list);
- kmem_free(dfp);
+ if (error == -EAGAIN) {
+ /*
+ * Caller wants a fresh transaction, so log a
+ * new log intent item to replace the old one
+ * and roll the transaction. See "Requesting
+ * a Fresh Transaction while Finishing
+ * Deferred Work" above.
+ */
+ dfp->dfp_intent = dfp->dfp_type->create_intent(*tp,
+ dfp->dfp_count);
+ dfp->dfp_done = NULL;
+ list_for_each(li, &dfp->dfp_work)
+ dfp->dfp_type->log_item(*tp, dfp->dfp_intent,
+ li);
+ } else {
+ /* Done with the dfp, free it. */
+ list_del(&dfp->dfp_list);
+ kmem_free(dfp);
+ }
if (cleanup_fn)
cleanup_fn(*tp, state, error);
projects / cascardo / linux.git / blobdiff