netfs: Change the read result collector to only use one work item

Change the way netfslib collects read results to do all the collection for
a particular read request using a single work item that walks along the
subrequest queue as subrequests make progress or complete, unlocking folios
progressively rather than doing the unlock in parallel as parallel requests
come in.

The code is remodelled to be more like the write-side code, though only
using a single stream.  This makes it more directly comparable and thus
easier to duplicate fixes between the two sides.

This has a number of advantages:

 (1) It's simpler.  There doesn't need to be a complex donation mechanism
     to handle mismatches between the size and alignment of subrequests and
     folios.  The collector unlocks folios as the subrequests covering each
     complete.

 (2) It should cause less scheduler overhead as there's a single work item
     in play unlocking pages in parallel when a read gets split up into a
     lot of subrequests instead of one per subrequest.

     Whilst the parallellism is nice in theory, in practice, the vast
     majority of loads are sequential reads of the whole file, so
     committing a bunch of threads to unlocking folios out of order doesn't
     help in those cases.

 (3) It should make it easier to implement content decryption.  A folio
     cannot be decrypted until all the requests that contribute to it have
     completed - and, again, most loads are sequential and so, most of the
     time, we want to begin decryption sequentially (though it's great if
     the decryption can happen in parallel).

There is a disadvantage in that we're losing the ability to decrypt and
unlock things on an as-things-arrive basis which may affect some
applications.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-28-dhowells@redhat.com
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 14 insertions, 7 deletions

diff --git a/fs/netfs/internal.h b/fs/netfs/internal.h
index e236f752af88..eb76f98c894b 100644
--- a/fs/netfs/internal.h
+++ b/fs/netfs/internal.h
@@ -82,20 +82,27 @@ static inline void netfs_see_request(struct netfs_io_request *rreq,
 	trace_netfs_rreq_ref(rreq->debug_id, refcount_read(&rreq->ref), what);
 }
 
+static inline void netfs_see_subrequest(struct netfs_io_subrequest *subreq,
+					enum netfs_sreq_ref_trace what)
+{
+	trace_netfs_sreq_ref(subreq->rreq->debug_id, subreq->debug_index,
+			     refcount_read(&subreq->ref), what);
+}
+
 /*
  * read_collect.c
  */
-void netfs_read_termination_worker(struct work_struct *work);
-void netfs_rreq_terminated(struct netfs_io_request *rreq);
+void netfs_read_collection_worker(struct work_struct *work);
+void netfs_wake_read_collector(struct netfs_io_request *rreq);
+void netfs_cache_read_terminated(void *priv, ssize_t transferred_or_error, bool was_async);
+ssize_t netfs_wait_for_read(struct netfs_io_request *rreq);
+void netfs_wait_for_pause(struct netfs_io_request *rreq);
 
 /*
  * read_pgpriv2.c
  */
-void netfs_pgpriv2_mark_copy_to_cache(struct netfs_io_subrequest *subreq,
-				      struct netfs_io_request *rreq,
-				      struct folio_queue *folioq,
-				      int slot);
-void netfs_pgpriv2_write_to_the_cache(struct netfs_io_request *rreq);
+void netfs_pgpriv2_copy_to_cache(struct netfs_io_request *rreq, struct folio *folio);
+void netfs_pgpriv2_end_copy_to_cache(struct netfs_io_request *rreq);
 bool netfs_pgpriv2_unlock_copied_folios(struct netfs_io_request *wreq);
 
 /*