[PATCH] netfs: Fix the handling of stream->front by removing it

[PATCH] netfs: Fix the handling of stream->front by removing it

[David Howells]

The netfs_io_stream::front member is meant to point to the subrequest
currently being collected on a stream, but it isn't actually used this way
by direct write (which mostly ignores it).  However, there's a tracepoint
which looks at it.  Further, stream->front is actually redundant with
stream->subrequests.next.

Fix the potential problem in the direct code by just removing the member
and using stream->subrequests.next instead, thereby also simplifying the
code.

Fixes: a0b4c7a49137 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
Reported-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/buffered_read.c     |    3 +--
 fs/netfs/direct_read.c       |    3 +--
 fs/netfs/direct_write.c      |    1 -
 fs/netfs/read_collect.c      |    4 ++--
 fs/netfs/read_single.c       |    1 -
 fs/netfs/write_collect.c     |    4 ++--
 fs/netfs/write_issue.c       |    3 +--
 include/linux/netfs.h        |    1 -
 include/trace/events/netfs.h |    8 ++++----
 9 files changed, 11 insertions(+), 17 deletions(-)

diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
index 88a0d801525f..a8c0d86118c5 100644
--- a/fs/netfs/buffered_read.c
+++ b/fs/netfs/buffered_read.c
@@ -171,9 +171,8 @@ static void netfs_queue_read(struct netfs_io_request *rreq,
 	spin_lock(&rreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-		stream->front = subreq;
 		if (!stream->active) {
-			stream->collected_to = stream->front->start;
+			stream->collected_to = subreq->start;
 			/* Store list pointers before active flag */
 			smp_store_release(&stream->active, true);
 		}
diff --git a/fs/netfs/direct_read.c b/fs/netfs/direct_read.c
index a498ee8d6674..f72e6da88cca 100644
--- a/fs/netfs/direct_read.c
+++ b/fs/netfs/direct_read.c
@@ -71,9 +71,8 @@ static int netfs_dispatch_unbuffered_reads(struct netfs_io_request *rreq)
 		spin_lock(&rreq->lock);
 		list_add_tail(&subreq->rreq_link, &stream->subrequests);
 		if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-			stream->front = subreq;
 			if (!stream->active) {
-				stream->collected_to = stream->front->start;
+				stream->collected_to = subreq->start;
 				/* Store list pointers before active flag */
 				smp_store_release(&stream->active, true);
 			}
diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
index 4d9760e36c11..f9ab69de3e29 100644
--- a/fs/netfs/direct_write.c
+++ b/fs/netfs/direct_write.c
@@ -111,7 +111,6 @@ static int netfs_unbuffered_write(struct netfs_io_request *wreq)
 			netfs_prepare_write(wreq, stream, wreq->start + wreq->transferred);
 			subreq = stream->construct;
 			stream->construct = NULL;
-			stream->front = NULL;
 		}
 
 		/* Check if (re-)preparation failed. */
diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
index 137f0e28a44c..e5f6665b3341 100644
--- a/fs/netfs/read_collect.c
+++ b/fs/netfs/read_collect.c
@@ -205,7 +205,8 @@ static void netfs_collect_read_results(struct netfs_io_request *rreq)
 	 * in progress.  The issuer thread may be adding stuff to the tail
 	 * whilst we're doing this.
 	 */
-	front = READ_ONCE(stream->front);
+	front = list_first_entry_or_null(&stream->subrequests,
+					 struct netfs_io_subrequest, rreq_link);
 	while (front) {
 		size_t transferred;
 
@@ -301,7 +302,6 @@ static void netfs_collect_read_results(struct netfs_io_request *rreq)
 		list_del_init(&front->rreq_link);
 		front = list_first_entry_or_null(&stream->subrequests,
 						 struct netfs_io_subrequest, rreq_link);
-		stream->front = front;
 		spin_unlock(&rreq->lock);
 		netfs_put_subrequest(remove,
 				     notes & ABANDON_SREQ ?
diff --git a/fs/netfs/read_single.c b/fs/netfs/read_single.c
index 8e6264f62a8f..d0e23bc42445 100644
--- a/fs/netfs/read_single.c
+++ b/fs/netfs/read_single.c
@@ -107,7 +107,6 @@ static int netfs_single_dispatch_read(struct netfs_io_request *rreq)
 	spin_lock(&rreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	trace_netfs_sreq(subreq, netfs_sreq_trace_added);
-	stream->front = subreq;
 	/* Store list pointers before active flag */
 	smp_store_release(&stream->active, true);
 	spin_unlock(&rreq->lock);
diff --git a/fs/netfs/write_collect.c b/fs/netfs/write_collect.c
index 83eb3dc1adf8..b194447f4b11 100644
--- a/fs/netfs/write_collect.c
+++ b/fs/netfs/write_collect.c
@@ -228,7 +228,8 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 		if (!smp_load_acquire(&stream->active))
 			continue;
 
-		front = stream->front;
+		front = list_first_entry_or_null(&stream->subrequests,
+						 struct netfs_io_subrequest, rreq_link);
 		while (front) {
 			trace_netfs_collect_sreq(wreq, front);
 			//_debug("sreq [%x] %llx %zx/%zx",
@@ -279,7 +280,6 @@ static void netfs_collect_write_results(struct netfs_io_request *wreq)
 			list_del_init(&front->rreq_link);
 			front = list_first_entry_or_null(&stream->subrequests,
 							 struct netfs_io_subrequest, rreq_link);
-			stream->front = front;
 			spin_unlock(&wreq->lock);
 			netfs_put_subrequest(remove,
 					     notes & SAW_FAILURE ?
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 437268f65640..2db688f94125 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -206,9 +206,8 @@ void netfs_prepare_write(struct netfs_io_request *wreq,
 	spin_lock(&wreq->lock);
 	list_add_tail(&subreq->rreq_link, &stream->subrequests);
 	if (list_is_first(&subreq->rreq_link, &stream->subrequests)) {
-		stream->front = subreq;
 		if (!stream->active) {
-			stream->collected_to = stream->front->start;
+			stream->collected_to = subreq->start;
 			/* Write list pointers before active flag */
 			smp_store_release(&stream->active, true);
 		}
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 72ee7d210a74..ba17ac5bf356 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -140,7 +140,6 @@ struct netfs_io_stream {
 	void (*issue_write)(struct netfs_io_subrequest *subreq);
 	/* Collection tracking */
 	struct list_head	subrequests;	/* Contributory I/O operations */
-	struct netfs_io_subrequest *front;	/* Op being collected */
 	unsigned long long	collected_to;	/* Position we've collected results to */
 	size_t			transferred;	/* The amount transferred from this stream */
 	unsigned short		error;		/* Aggregate error for the stream */
diff --git a/include/trace/events/netfs.h b/include/trace/events/netfs.h
index 2d366be46a1c..cbe28211106c 100644
--- a/include/trace/events/netfs.h
+++ b/include/trace/events/netfs.h
@@ -740,19 +740,19 @@ TRACE_EVENT(netfs_collect_stream,
 		    __field(unsigned int,	wreq)
 		    __field(unsigned char,	stream)
 		    __field(unsigned long long,	collected_to)
-		    __field(unsigned long long,	front)
+		    __field(unsigned long long,	issued_to)
 			     ),
 
 	    TP_fast_assign(
 		    __entry->wreq	= wreq->debug_id;
 		    __entry->stream	= stream->stream_nr;
 		    __entry->collected_to = stream->collected_to;
-		    __entry->front	= stream->front ? stream->front->start : UINT_MAX;
+		    __entry->issued_to	= atomic64_read(&wreq->issued_to);
 			   ),
 
-	    TP_printk("R=%08x[%x:] cto=%llx frn=%llx",
+	    TP_printk("R=%08x[%x:] cto=%llx ito=%llx",
 		      __entry->wreq, __entry->stream,
-		      __entry->collected_to, __entry->front)
+		      __entry->collected_to, __entry->issued_to)
 	    );
 
 TRACE_EVENT(netfs_folioq,

Re: [PATCH] netfs: Fix the handling of stream->front by removing it

[Paulo Alcantara]

David Howells <dhowells@redhat.com> writes:

> The netfs_io_stream::front member is meant to point to the subrequest
> currently being collected on a stream, but it isn't actually used this way
> by direct write (which mostly ignores it).  However, there's a tracepoint
> which looks at it.  Further, stream->front is actually redundant with
> stream->subrequests.next.
>
> Fix the potential problem in the direct code by just removing the member
> and using stream->subrequests.next instead, thereby also simplifying the
> code.
>
> Fixes: a0b4c7a49137 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
> Reported-by: Paulo Alcantara <pc@manguebit.org>
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/netfs/buffered_read.c     |    3 +--
>  fs/netfs/direct_read.c       |    3 +--
>  fs/netfs/direct_write.c      |    1 -
>  fs/netfs/read_collect.c      |    4 ++--
>  fs/netfs/read_single.c       |    1 -
>  fs/netfs/write_collect.c     |    4 ++--
>  fs/netfs/write_issue.c       |    3 +--
>  include/linux/netfs.h        |    1 -
>  include/trace/events/netfs.h |    8 ++++----
>  9 files changed, 11 insertions(+), 17 deletions(-)

Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>

Re: [PATCH] netfs: Fix the handling of stream->front by removing it

[Christian Brauner]

On Wed, 25 Mar 2026 08:20:17 +0000, David Howells wrote:
> The netfs_io_stream::front member is meant to point to the subrequest
> currently being collected on a stream, but it isn't actually used this way
> by direct write (which mostly ignores it).  However, there's a tracepoint
> which looks at it.  Further, stream->front is actually redundant with
> stream->subrequests.next.
> 
> Fix the potential problem in the direct code by just removing the member
> and using stream->subrequests.next instead, thereby also simplifying the
> code.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] netfs: Fix the handling of stream->front by removing it
      https://git.kernel.org/vfs/vfs/c/0e764b9d4607