netfs: Add a function to consolidate beginning a read

Add a function to do the steps needed to begin a read request, allowing
this code to be removed from several other functions and consolidated.

Changes
=======
ver #2)
 - Move before the unstaticking patch so that some functions can be left
   static.
 - Set uninitialised return code in netfs_begin_read()[1][2].
 - Fixed a refleak caused by non-removal of a get from netfs_write_begin()
   when the request submission code got moved to netfs_begin_read().

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20220303163826.1120936-1-nathan@kernel.org/ [1]
Link: https://lore.kernel.org/r/20220303235647.1297171-1-colin.i.king@gmail.com/ [2]
Link: https://lore.kernel.org/r/164623004355.3564931.7275693529042495641.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/164678214287.1200972.16734134007649832160.stgit@warthog.procyon.org.uk/ # v2

Author: dhowells

fs/netfs/internal.h

@@ -39,7 +39,7 @@ static inline void netfs_see_request(struct netfs_io_request *rreq,
  */
 extern unsigned int netfs_debug;
 
-void netfs_rreq_work(struct work_struct *work);
+int netfs_begin_read(struct netfs_io_request *rreq, bool sync);
 
 /*
  * stats.c

fs/netfs/objects.c

@@ -35,7 +35,7 @@ struct netfs_io_request *netfs_alloc_request(struct address_space *mapping,
 	rreq->i_size	= i_size_read(inode);
 	rreq->debug_id	= atomic_inc_return(&debug_ids);
 	INIT_LIST_HEAD(&rreq->subrequests);
-	INIT_WORK(&rreq->work, netfs_rreq_work);
+	INIT_WORK(&rreq->work, NULL);
 	refcount_set(&rreq->ref, 1);
 	__set_bit(NETFS_RREQ_IN_PROGRESS, &rreq->flags);
 	if (rreq->netfs_ops->init_request) {

fs/netfs/read_helper.c

@@ -443,7 +443,7 @@ static void netfs_rreq_assess(struct netfs_io_request *rreq, bool was_async)
 	netfs_rreq_completed(rreq, was_async);
 }
 
-void netfs_rreq_work(struct work_struct *work)
+static void netfs_rreq_work(struct work_struct *work)
 {
 	struct netfs_io_request *rreq =
 		container_of(work, struct netfs_io_request, work);
@@ -688,6 +688,69 @@ static bool netfs_rreq_submit_slice(struct netfs_io_request *rreq,
 	return false;
 }
 
+/*
+ * Begin the process of reading in a chunk of data, where that data may be
+ * stitched together from multiple sources, including multiple servers and the
+ * local cache.
+ */
+int netfs_begin_read(struct netfs_io_request *rreq, bool sync)
+{
+	unsigned int debug_index = 0;
+	int ret;
+
+	_enter("R=%x %llx-%llx",
+	       rreq->debug_id, rreq->start, rreq->start + rreq->len - 1);
+
+	if (rreq->len == 0) {
+		pr_err("Zero-sized read [R=%x]\n", rreq->debug_id);
+		netfs_put_request(rreq, false, netfs_rreq_trace_put_zero_len);
+		return -EIO;
+	}
+
+	rreq->work.func = netfs_rreq_work;
+
+	if (sync)
+		netfs_get_request(rreq, netfs_rreq_trace_get_hold);
+
+	/* Chop the read into slices according to what the cache and the netfs
+	 * want and submit each one.
+	 */
+	atomic_set(&rreq->nr_outstanding, 1);
+	do {
+		if (!netfs_rreq_submit_slice(rreq, &debug_index))
+			break;
+
+	} while (rreq->submitted < rreq->len);
+
+	if (sync) {
+		/* Keep nr_outstanding incremented so that the ref always belongs to
+		 * us, and the service code isn't punted off to a random thread pool to
+		 * process.
+		 */
+		for (;;) {
+			wait_var_event(&rreq->nr_outstanding,
+				       atomic_read(&rreq->nr_outstanding) == 1);
+			netfs_rreq_assess(rreq, false);
+			if (!test_bit(NETFS_RREQ_IN_PROGRESS, &rreq->flags))
+				break;
+			cond_resched();
+		}
+
+		ret = rreq->error;
+		if (ret == 0 && rreq->submitted < rreq->len) {
+			trace_netfs_failure(rreq, NULL, ret, netfs_fail_short_read);
+			ret = -EIO;
+		}
+		netfs_put_request(rreq, false, netfs_rreq_trace_put_hold);
+	} else {
+		/* If we decrement nr_outstanding to 0, the ref belongs to us. */
+		if (atomic_dec_and_test(&rreq->nr_outstanding))
+			netfs_rreq_assess(rreq, false);
+		ret = 0;
+	}
+	return ret;
+}
+
 static void netfs_cache_expand_readahead(struct netfs_io_request *rreq,
 					 loff_t *_start, size_t *_len, loff_t i_size)
 {
@@ -750,7 +813,6 @@ void netfs_readahead(struct readahead_control *ractl)
 {
 	struct netfs_io_request *rreq;
 	struct netfs_i_context *ctx = netfs_i_context(ractl->mapping->host);
-	unsigned int debug_index = 0;
 	int ret;
 
 	_enter("%lx,%x", readahead_index(ractl), readahead_count(ractl));
@@ -777,22 +839,13 @@ void netfs_readahead(struct readahead_control *ractl)
 
 	netfs_rreq_expand(rreq, ractl);
 
-	atomic_set(&rreq->nr_outstanding, 1);
-	do {
-		if (!netfs_rreq_submit_slice(rreq, &debug_index))
-			break;
-
-	} while (rreq->submitted < rreq->len);
-
 	/* Drop the refs on the folios here rather than in the cache or
 	 * filesystem.  The locks will be dropped in netfs_rreq_unlock().
 	 */
 	while (readahead_folio(ractl))
 		;
 
-	/* If we decrement nr_outstanding to 0, the ref belongs to us. */
-	if (atomic_dec_and_test(&rreq->nr_outstanding))
-		netfs_rreq_assess(rreq, false);
+	netfs_begin_read(rreq, false);
 	return;
 
 cleanup_free:
@@ -821,7 +874,6 @@ int netfs_readpage(struct file *file, struct page *subpage)
 	struct address_space *mapping = folio->mapping;
 	struct netfs_io_request *rreq;
 	struct netfs_i_context *ctx = netfs_i_context(mapping->host);
-	unsigned int debug_index = 0;
 	int ret;
 
 	_enter("%lx", folio_index(folio));
@@ -836,42 +888,16 @@ int netfs_readpage(struct file *file, struct page *subpage)
 
 	if (ctx->ops->begin_cache_operation) {
 		ret = ctx->ops->begin_cache_operation(rreq);
-		if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS) {
-			folio_unlock(folio);
-			goto out;
-		}
+		if (ret == -ENOMEM || ret == -EINTR || ret == -ERESTARTSYS)
+			goto discard;
 	}
 
 	netfs_stat(&netfs_n_rh_readpage);
 	trace_netfs_read(rreq, rreq->start, rreq->len, netfs_read_trace_readpage);
+	return netfs_begin_read(rreq, true);
 
-	netfs_get_request(rreq, netfs_rreq_trace_get_hold);
-
-	atomic_set(&rreq->nr_outstanding, 1);
-	do {
-		if (!netfs_rreq_submit_slice(rreq, &debug_index))
-			break;
-
-	} while (rreq->submitted < rreq->len);
-
-	/* Keep nr_outstanding incremented so that the ref always belongs to us, and
-	 * the service code isn't punted off to a random thread pool to
-	 * process.
-	 */
-	do {
-		wait_var_event(&rreq->nr_outstanding,
-			       atomic_read(&rreq->nr_outstanding) == 1);
-		netfs_rreq_assess(rreq, false);
-	} while (test_bit(NETFS_RREQ_IN_PROGRESS, &rreq->flags));
-
-	ret = rreq->error;
-	if (ret == 0 && rreq->submitted < rreq->len) {
-		trace_netfs_failure(rreq, NULL, ret, netfs_fail_short_readpage);
-		ret = -EIO;
-	}
-out:
-	netfs_put_request(rreq, false, netfs_rreq_trace_put_hold);
-	return ret;
+discard:
+	netfs_put_request(rreq, false, netfs_rreq_trace_put_discard);
 alloc_error:
 	folio_unlock(folio);
 	return ret;
@@ -966,7 +992,7 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
 	struct netfs_io_request *rreq;
 	struct netfs_i_context *ctx = netfs_i_context(file_inode(file ));
 	struct folio *folio;
-	unsigned int debug_index = 0, fgp_flags;
+	unsigned int fgp_flags;
 	pgoff_t index = pos >> PAGE_SHIFT;
 	int ret;
 
@@ -1029,39 +1055,13 @@ int netfs_write_begin(struct file *file, struct address_space *mapping,
 	 */
 	ractl._nr_pages = folio_nr_pages(folio);
 	netfs_rreq_expand(rreq, &ractl);
-	netfs_get_request(rreq, netfs_rreq_trace_get_hold);
 
 	/* We hold the folio locks, so we can drop the references */
 	folio_get(folio);
 	while (readahead_folio(&ractl))
 		;
 
-	atomic_set(&rreq->nr_outstanding, 1);
-	do {
-		if (!netfs_rreq_submit_slice(rreq, &debug_index))
-			break;
-
-	} while (rreq->submitted < rreq->len);
-
-	/* Keep nr_outstanding incremented so that the ref always belongs to
-	 * us, and the service code isn't punted off to a random thread pool to
-	 * process.
-	 */
-	for (;;) {
-		wait_var_event(&rreq->nr_outstanding,
-			       atomic_read(&rreq->nr_outstanding) == 1);
-		netfs_rreq_assess(rreq, false);
-		if (!test_bit(NETFS_RREQ_IN_PROGRESS, &rreq->flags))
-			break;
-		cond_resched();
-	}
-
-	ret = rreq->error;
-	if (ret == 0 && rreq->submitted < rreq->len) {
-		trace_netfs_failure(rreq, NULL, ret, netfs_fail_short_write_begin);
-		ret = -EIO;
-	}
-	netfs_put_request(rreq, false, netfs_rreq_trace_put_hold);
+	ret = netfs_begin_read(rreq, true);
 	if (ret < 0)
 		goto error;
 

include/trace/events/netfs.h

@@ -56,17 +56,18 @@
 	EM(netfs_fail_check_write_begin,	"check-write-begin")	\
 	EM(netfs_fail_copy_to_cache,		"copy-to-cache")	\
 	EM(netfs_fail_read,			"read")			\
-	EM(netfs_fail_short_readpage,		"short-readpage")	\
-	EM(netfs_fail_short_write_begin,	"short-write-begin")	\
+	EM(netfs_fail_short_read,		"short-read")		\
 	E_(netfs_fail_prepare_write,		"prep-write")
 
 #define netfs_rreq_ref_traces					\
 	EM(netfs_rreq_trace_get_hold,		"GET HOLD   ")	\
 	EM(netfs_rreq_trace_get_subreq,		"GET SUBREQ ")	\
 	EM(netfs_rreq_trace_put_complete,	"PUT COMPLT ")	\
+	EM(netfs_rreq_trace_put_discard,	"PUT DISCARD")	\
 	EM(netfs_rreq_trace_put_failed,		"PUT FAILED ")	\
 	EM(netfs_rreq_trace_put_hold,		"PUT HOLD   ")	\
 	EM(netfs_rreq_trace_put_subreq,		"PUT SUBREQ ")	\
+	EM(netfs_rreq_trace_put_zero_len,	"PUT ZEROLEN")	\
 	E_(netfs_rreq_trace_new,		"NEW        ")
 
 #define netfs_sreq_ref_traces					\