netfs: zero the tail of a short DIO/unbuffered readgrafted

The buffered read collector zero-fills the tail of a short read that
stops below the inode's i_size (netfs_clear_unread()), so a read that
races an extending write still returns the expected number of bytes.
The non-buffered collector path does no such thing: it just records
how much was transferred.

Add the same zero-fill for the non-buffered case, gated on
NETFS_SREQ_CLEAR_TAIL: a subreq's source sets that flag when a short
result from it is known to be safe to treat as a hole, as opposed to
NETFS_SREQ_HIT_EOF, which means the read genuinely ran off the end of
the file and should be reported short as-is. Only CLEAR_TAIL should
zero-fill here; a real EOF must stay a real short read.

No source currently sets CLEAR_TAIL on an unbuffered/DIO subrequest,
so this is inert on its own -- a following change teaches cifs to set
it in the one case that needs it.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org

1 files changed, 138 insertions, 0 deletions

diff --git a/drivers/md/dm-path-selector.c b/drivers/md/dm-path-selector.c
new file mode 100644
index 000000000..006a7991f
--- /dev/null
+++ b/drivers/md/dm-path-selector.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2003 Sistina Software.
+ * Copyright (C) 2004 Red Hat, Inc. All rights reserved.
+ *
+ * Module Author: Heinz Mauelshagen
+ *
+ * This file is released under the GPL.
+ *
+ * Path selector registration.
+ */
+
+#include <linux/device-mapper.h>
+#include <linux/module.h>
+
+#include "dm-path-selector.h"
+
+#include <linux/slab.h>
+
+struct ps_internal {
+	struct path_selector_type pst;
+	struct list_head list;
+};
+
+#define pst_to_psi(__pst) container_of((__pst), struct ps_internal, pst)
+
+static LIST_HEAD(_path_selectors);
+static DECLARE_RWSEM(_ps_lock);
+
+static struct ps_internal *__find_path_selector_type(const char *name)
+{
+	struct ps_internal *psi;
+
+	list_for_each_entry(psi, &_path_selectors, list) {
+		if (!strcmp(name, psi->pst.name))
+			return psi;
+	}
+
+	return NULL;
+}
+
+static struct ps_internal *get_path_selector(const char *name)
+{
+	struct ps_internal *psi;
+
+	down_read(&_ps_lock);
+	psi = __find_path_selector_type(name);
+	if (psi && !try_module_get(psi->pst.module))
+		psi = NULL;
+	up_read(&_ps_lock);
+
+	return psi;
+}
+
+struct path_selector_type *dm_get_path_selector(const char *name)
+{
+	struct ps_internal *psi;
+
+	if (!name)
+		return NULL;
+
+	psi = get_path_selector(name);
+	if (!psi) {
+		request_module("dm-%s", name);
+		psi = get_path_selector(name);
+	}
+
+	return psi ? &psi->pst : NULL;
+}
+
+void dm_put_path_selector(struct path_selector_type *pst)
+{
+	struct ps_internal *psi;
+
+	if (!pst)
+		return;
+
+	down_read(&_ps_lock);
+	psi = __find_path_selector_type(pst->name);
+	if (!psi)
+		goto out;
+
+	module_put(psi->pst.module);
+out:
+	up_read(&_ps_lock);
+}
+
+static struct ps_internal *_alloc_path_selector(struct path_selector_type *pst)
+{
+	struct ps_internal *psi = kzalloc_obj(*psi);
+
+	if (psi)
+		psi->pst = *pst;
+
+	return psi;
+}
+
+int dm_register_path_selector(struct path_selector_type *pst)
+{
+	int r = 0;
+	struct ps_internal *psi = _alloc_path_selector(pst);
+
+	if (!psi)
+		return -ENOMEM;
+
+	down_write(&_ps_lock);
+
+	if (__find_path_selector_type(pst->name)) {
+		kfree(psi);
+		r = -EBUSY;
+	} else
+		list_add(&psi->list, &_path_selectors);
+
+	up_write(&_ps_lock);
+
+	return r;
+}
+EXPORT_SYMBOL_GPL(dm_register_path_selector);
+
+void dm_unregister_path_selector(struct path_selector_type *pst)
+{
+	struct ps_internal *psi;
+
+	down_write(&_ps_lock);
+
+	psi = __find_path_selector_type(pst->name);
+	if (WARN_ON(!psi)) {
+		up_write(&_ps_lock);
+		return;
+	}
+
+	list_del(&psi->list);
+
+	up_write(&_ps_lock);
+
+	kfree(psi);
+}
+EXPORT_SYMBOL_GPL(dm_unregister_path_selector);