netfs: Simplify the writeback code

Use the new folio_queue structures to simplify the writeback code.  The
problem with referring to the i_pages xarray directly is that we may have
gaps in the sequence of folios we're writing from that we need to skip when
we're removing the writeback mark from the folios we're writing back from.

At the moment the code tries to deal with this by carefully tracking the
gaps in each writeback stream (eg. write to server and write to cache) and
divining when there's a gap that spans folios (something that's not helped
by folios not being a consistent size).

Instead, the folio_queue buffer contains pointers only the folios we're
dealing with, has them in ascending order and indicates a gap by placing
non-consequitive folios next to each other.  This makes it possible to
track where we need to clean up to by just keeping track of where we've
processed to on each stream and taking the minimum.

Note that the I/O iterator is always rounded up to the end of the folio,
even if that is beyond the EOF position, so that the cache can do DIO from
the page.  The excess space is cleared, though mmapped writes clobber it.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240814203850.2240469-18-dhowells@redhat.com/ # v2
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: dhowells

fs/netfs/write_collect.c

@@ -15,15 +15,11 @@
 
 /* Notes made in the collector */
 #define HIT_PENDING		0x01	/* A front op was still pending */
-#define SOME_EMPTY		0x02	/* One of more streams are empty */
-#define ALL_EMPTY		0x04	/* All streams are empty */
-#define MAYBE_DISCONTIG		0x08	/* A front op may be discontiguous (rounded to PAGE_SIZE) */
-#define NEED_REASSESS		0x10	/* Need to loop round and reassess */
-#define REASSESS_DISCONTIG	0x20	/* Reassess discontiguity if contiguity advances */
-#define MADE_PROGRESS		0x40	/* Made progress cleaning up a stream or the folio set */
-#define BUFFERED		0x80	/* The pagecache needs cleaning up */
-#define NEED_RETRY		0x100	/* A front op requests retrying */
-#define SAW_FAILURE		0x200	/* One stream or hit a permanent failure */
+#define NEED_REASSESS		0x02	/* Need to loop round and reassess */
+#define MADE_PROGRESS		0x04	/* Made progress cleaning up a stream or the folio set */
+#define BUFFERED		0x08	/* The pagecache needs cleaning up */
+#define NEED_RETRY		0x10	/* A front op requests retrying */
+#define SAW_FAILURE		0x20	/* One stream or hit a permanent failure */
 
 /*
  * Successful completion of write of a folio to the server and/or cache.  Note
@@ -85,10 +81,10 @@ int netfs_folio_written_back(struct folio *folio)
  * Unlock any folios we've finished with.
  */
 static void netfs_writeback_unlock_folios(struct netfs_io_request *wreq,
-					  unsigned long long collected_to,
 					  unsigned int *notes)
 {
 	struct folio_queue *folioq = wreq->buffer;
+	unsigned long long collected_to = wreq->collected_to;
 	unsigned int slot = wreq->buffer_head_slot;
 
 	if (slot >= folioq_nr_slots(folioq)) {
@@ -117,12 +113,6 @@ static void netfs_writeback_unlock_folios(struct netfs_io_request *wreq,
 
 		trace_netfs_collect_folio(wreq, folio, fend, collected_to);
 
-		if (fpos + fsize > wreq->contiguity) {
-			trace_netfs_collect_contig(wreq, fpos + fsize,
-						   netfs_contig_trace_unlock);
-			wreq->contiguity = fpos + fsize;
-		}
-
 		/* Unlock any folio we've transferred all of. */
 		if (collected_to < fend)
 			break;
@@ -380,7 +370,7 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 {
 	struct netfs_io_subrequest *front, *remove;
 	struct netfs_io_stream *stream;
-	unsigned long long collected_to;
+	unsigned long long collected_to, issued_to;
 	unsigned int notes;
 	int s;
 
@@ -389,28 +379,21 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 	trace_netfs_rreq(wreq, netfs_rreq_trace_collect);
 
 reassess_streams:
+	issued_to = atomic64_read(&wreq->issued_to);
 	smp_rmb();
 	collected_to = ULLONG_MAX;
-	if (wreq->origin == NETFS_WRITEBACK)
-		notes = ALL_EMPTY | BUFFERED | MAYBE_DISCONTIG;
-	else if (wreq->origin == NETFS_WRITETHROUGH)
-		notes = ALL_EMPTY | BUFFERED;
+	if (wreq->origin == NETFS_WRITEBACK ||
+	    wreq->origin == NETFS_WRITETHROUGH)
+		notes = BUFFERED;
 	else
-		notes = ALL_EMPTY;
+		notes = 0;
 
 	/* Remove completed subrequests from the front of the streams and
 	 * advance the completion point on each stream.  We stop when we hit
 	 * something that's in progress.  The issuer thread may be adding stuff
 	 * to the tail whilst we're doing this.
-	 *
-	 * We must not, however, merge in discontiguities that span whole
-	 * folios that aren't under writeback.  This is made more complicated
-	 * by the folios in the gap being of unpredictable sizes - if they even
-	 * exist - but we don't want to look them up.
 	 */
 	for (s = 0; s < NR_IO_STREAMS; s++) {
-		loff_t rstart, rend;
-
 		stream = &wreq->io_streams[s];
 		/* Read active flag before list pointers */
 		if (!smp_load_acquire(&stream->active))
@@ -422,26 +405,10 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 			//_debug("sreq [%x] %llx %zx/%zx",
 			//       front->debug_index, front->start, front->transferred, front->len);
 
-			/* Stall if there may be a discontinuity. */
-			rstart = round_down(front->start, PAGE_SIZE);
-			if (rstart > wreq->contiguity) {
-				if (wreq->contiguity > stream->collected_to) {
-					trace_netfs_collect_gap(wreq, stream,
-								wreq->contiguity, 'D');
-					stream->collected_to = wreq->contiguity;
-				}
-				notes |= REASSESS_DISCONTIG;
-				break;
+			if (stream->collected_to < front->start) {
+				trace_netfs_collect_gap(wreq, stream, issued_to, 'F');
+				stream->collected_to = front->start;
 			}
-			rend = round_up(front->start + front->len, PAGE_SIZE);
-			if (rend > wreq->contiguity) {
-				trace_netfs_collect_contig(wreq, rend,
-							   netfs_contig_trace_collect);
-				wreq->contiguity = rend;
-				if (notes & REASSESS_DISCONTIG)
-					notes |= NEED_REASSESS;
-			}
-			notes &= ~MAYBE_DISCONTIG;
 
 			/* Stall if the front is still undergoing I/O. */
 			if (test_bit(NETFS_SREQ_IN_PROGRESS, &front->flags)) {
@@ -483,26 +450,20 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 			front = list_first_entry_or_null(&stream->subrequests,
 							 struct netfs_io_subrequest, rreq_link);
 			stream->front = front;
-			if (!front) {
-				unsigned long long jump_to = atomic64_read(&wreq->issued_to);
-
-				if (stream->collected_to < jump_to) {
-					trace_netfs_collect_gap(wreq, stream, jump_to, 'A');
-					stream->collected_to = jump_to;
-				}
-			}
-
 			spin_unlock_bh(&wreq->lock);
 			netfs_put_subrequest(remove, false,
 					     notes & SAW_FAILURE ?
 					     netfs_sreq_trace_put_cancel :
 					     netfs_sreq_trace_put_done);
 		}
 
-		if (front)
-			notes &= ~ALL_EMPTY;
-		else
-			notes |= SOME_EMPTY;
+		/* If we have an empty stream, we need to jump it forward
+		 * otherwise the collection point will never advance.
+		 */
+		if (!front && issued_to > stream->collected_to) {
+			trace_netfs_collect_gap(wreq, stream, issued_to, 'E');
+			stream->collected_to = issued_to;
+		}
 
 		if (stream->collected_to < collected_to)
 			collected_to = stream->collected_to;
@@ -511,36 +472,6 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 	if (collected_to != ULLONG_MAX && collected_to > wreq->collected_to)
 		wreq->collected_to = collected_to;
 
-	/* If we have an empty stream, we need to jump it forward over any gap
-	 * otherwise the collection point will never advance.
-	 *
-	 * Note that the issuer always adds to the stream with the lowest
-	 * so-far submitted start, so if we see two consecutive subreqs in one
-	 * stream with nothing between then in another stream, then the second
-	 * stream has a gap that can be jumped.
-	 */
-	if (notes & SOME_EMPTY) {
-		unsigned long long jump_to = wreq->start + READ_ONCE(wreq->submitted);
-
-		for (s = 0; s < NR_IO_STREAMS; s++) {
-			stream = &wreq->io_streams[s];
-			if (stream->active &&
-			    stream->front &&
-			    stream->front->start < jump_to)
-				jump_to = stream->front->start;
-		}
-
-		for (s = 0; s < NR_IO_STREAMS; s++) {
-			stream = &wreq->io_streams[s];
-			if (stream->active &&
-			    !stream->front &&
-			    stream->collected_to < jump_to) {
-				trace_netfs_collect_gap(wreq, stream, jump_to, 'B');
-				stream->collected_to = jump_to;
-			}
-		}
-	}
-
 	for (s = 0; s < NR_IO_STREAMS; s++) {
 		stream = &wreq->io_streams[s];
 		if (stream->active)
@@ -551,43 +482,14 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 
 	/* Unlock any folios that we have now finished with. */
 	if (notes & BUFFERED) {
-		unsigned long long clean_to = min(wreq->collected_to, wreq->contiguity);
-
-		if (wreq->cleaned_to < clean_to)
-			netfs_writeback_unlock_folios(wreq, clean_to, &notes);
+		if (wreq->cleaned_to < wreq->collected_to)
+			netfs_writeback_unlock_folios(wreq, &notes);
 	} else {
 		wreq->cleaned_to = wreq->collected_to;
 	}
 
 	// TODO: Discard encryption buffers
 
-	/* If all streams are discontiguous with the last folio we cleared, we
-	 * may need to skip a set of folios.
-	 */
-	if ((notes & (MAYBE_DISCONTIG | ALL_EMPTY)) == MAYBE_DISCONTIG) {
-		unsigned long long jump_to = ULLONG_MAX;
-
-		for (s = 0; s < NR_IO_STREAMS; s++) {
-			stream = &wreq->io_streams[s];
-			if (stream->active && stream->front &&
-			    stream->front->start < jump_to)
-				jump_to = stream->front->start;
-		}
-
-		trace_netfs_collect_contig(wreq, jump_to, netfs_contig_trace_jump);
-		wreq->contiguity = jump_to;
-		wreq->cleaned_to = jump_to;
-		wreq->collected_to = jump_to;
-		for (s = 0; s < NR_IO_STREAMS; s++) {
-			stream = &wreq->io_streams[s];
-			if (stream->collected_to < jump_to)
-				stream->collected_to = jump_to;
-		}
-		//cond_resched();
-		notes |= MADE_PROGRESS;
-		goto reassess_streams;
-	}
-
 	if (notes & NEED_RETRY)
 		goto need_retry;
 	if ((notes & MADE_PROGRESS) && test_bit(NETFS_RREQ_PAUSE, &wreq->flags)) {

fs/netfs/write_issue.c

@@ -107,7 +107,6 @@ struct netfs_io_request *netfs_create_write_req(struct address_space *mapping,
 	if (is_buffered && netfs_is_cache_enabled(ictx))
 		fscache_begin_write_operation(&wreq->cache_resources, netfs_i_cookie(ictx));
 
-	wreq->contiguity = wreq->start;
 	wreq->cleaned_to = wreq->start;
 
 	wreq->io_streams[0].stream_nr		= 0;
@@ -158,6 +157,7 @@ static void netfs_prepare_write(struct netfs_io_request *wreq,
 	subreq->source		= stream->source;
 	subreq->start		= start;
 	subreq->stream_nr	= stream->stream_nr;
+	subreq->io_iter		= wreq->io_iter;
 
 	_enter("R=%x[%x]", wreq->debug_id, subreq->debug_index);
 
@@ -213,22 +213,15 @@ static void netfs_prepare_write(struct netfs_io_request *wreq,
  * netfs_write_subrequest_terminated() when complete.
  */
 static void netfs_do_issue_write(struct netfs_io_stream *stream,
-				 struct netfs_io_subrequest *subreq,
-				 struct iov_iter *source)
+				 struct netfs_io_subrequest *subreq)
 {
 	struct netfs_io_request *wreq = subreq->rreq;
-	size_t size = subreq->len - subreq->transferred;
 
 	_enter("R=%x[%x],%zx", wreq->debug_id, subreq->debug_index, subreq->len);
 
 	if (test_bit(NETFS_SREQ_FAILED, &subreq->flags))
 		return netfs_write_subrequest_terminated(subreq, subreq->error, false);
 
-	// TODO: Use encrypted buffer
-	subreq->io_iter = *source;
-	iov_iter_advance(source, size);
-	iov_iter_truncate(&subreq->io_iter, size);
-
 	trace_netfs_sreq(subreq, netfs_sreq_trace_submit);
 	stream->issue_write(subreq);
 }
@@ -237,8 +230,15 @@ void netfs_reissue_write(struct netfs_io_stream *stream,
 			 struct netfs_io_subrequest *subreq,
 			 struct iov_iter *source)
 {
+	size_t size = subreq->len - subreq->transferred;
+
+	// TODO: Use encrypted buffer
+	subreq->io_iter = *source;
+	iov_iter_advance(source, size);
+	iov_iter_truncate(&subreq->io_iter, size);
+
 	__set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);
-	netfs_do_issue_write(stream, subreq, source);
+	netfs_do_issue_write(stream, subreq);
 }
 
 static void netfs_issue_write(struct netfs_io_request *wreq,
@@ -249,10 +249,8 @@ static void netfs_issue_write(struct netfs_io_request *wreq,
 	if (!subreq)
 		return;
 	stream->construct = NULL;
-
-	if (subreq->start + subreq->len > wreq->start + wreq->submitted)
-		WRITE_ONCE(wreq->submitted, subreq->start + subreq->len - wreq->start);
-	netfs_do_issue_write(stream, subreq, &wreq->io_iter);
+	subreq->io_iter.count = subreq->len;
+	netfs_do_issue_write(stream, subreq);
 }
 
 /*
@@ -464,10 +462,11 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 		if (choose_s < 0)
 			break;
 		stream = &wreq->io_streams[choose_s];
+		wreq->io_iter.iov_offset = stream->submit_off;
 
+		atomic64_set(&wreq->issued_to, fpos + stream->submit_off);
 		part = netfs_advance_write(wreq, stream, fpos + stream->submit_off,
 					   stream->submit_len, to_eof);
-		atomic64_set(&wreq->issued_to, fpos + stream->submit_off);
 		stream->submit_off += part;
 		stream->submit_max_len -= part;
 		if (part > stream->submit_len)
@@ -478,6 +477,8 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 			debug = true;
 	}
 
+	wreq->io_iter.iov_offset = 0;
+	iov_iter_advance(&wreq->io_iter, fsize);
 	atomic64_set(&wreq->issued_to, fpos + fsize);
 
 	if (!debug)
@@ -526,10 +527,10 @@ int netfs_writepages(struct address_space *mapping,
 	netfs_stat(&netfs_n_wh_writepages);
 
 	do {
-		_debug("wbiter %lx %llx", folio->index, wreq->start + wreq->submitted);
+		_debug("wbiter %lx %llx", folio->index, atomic64_read(&wreq->issued_to));
 
 		/* It appears we don't have to handle cyclic writeback wrapping. */
-		WARN_ON_ONCE(wreq && folio_pos(folio) < wreq->start + wreq->submitted);
+		WARN_ON_ONCE(wreq && folio_pos(folio) < atomic64_read(&wreq->issued_to));
 
 		if (netfs_folio_group(folio) != NETFS_FOLIO_COPY_TO_CACHE &&
 		    unlikely(!test_bit(NETFS_RREQ_UPLOAD_TO_SERVER, &wreq->flags))) {
@@ -673,6 +674,7 @@ int netfs_unbuffered_write(struct netfs_io_request *wreq, bool may_wait, size_t
 		part = netfs_advance_write(wreq, upload, start, len, false);
 		start += part;
 		len -= part;
+		iov_iter_advance(&wreq->io_iter, part);
 		if (test_bit(NETFS_RREQ_PAUSE, &wreq->flags)) {
 			trace_netfs_rreq(wreq, netfs_rreq_trace_wait_pause);
 			wait_on_bit(&wreq->flags, NETFS_RREQ_PAUSE, TASK_UNINTERRUPTIBLE);

include/linux/netfs.h

@@ -257,7 +257,6 @@ struct netfs_io_request {
 	unsigned long long	i_size;		/* Size of the file */
 	unsigned long long	start;		/* Start position */
 	atomic64_t		issued_to;	/* Write issuer folio cursor */
-	unsigned long long	contiguity;	/* Tracking for gaps in the writeback sequence */
 	unsigned long long	collected_to;	/* Point we've collected to */
 	unsigned long long	cleaned_to;	/* Position we've cleaned folios to */
 	pgoff_t			no_unlock_folio; /* Don't unlock this folio after read */