[Devel] [PATCH hci-8.1 5/5] dm/push_backup: add event file descriptor interface

[Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>]

The backup tool currently waits for the next cluster range to back up
by sleeping inside the "push_backup_read" target message. target_message()
calls into the target under dm_get_live_table(), i.e. with md->io_barrier
SRCU read-locked, so for the whole time the tool waits for a write to be
intercepted anything doing synchronize_srcu(&md->io_barrier) on this device
is stuck: dm suspend, table reload/remove and in particular
dm_internal_suspend_fast() called by "@stats_create". An unrelated dm-stats
consumer therefore hangs together with everything that later queues behind
md->suspend_lock.

Add a file descriptor based interface that does not involve dm messages
on the data path:

  push_backup_get_fd        - returns an anon inode fd in the message
                              result
  read(fd, &range, 16)      - next struct push_backup_range to back up;
                              0 when the backup is finished,
                              -ESTALE when not running,
                              -ENODEV after the target is destroyed
  write(fd, &range, 16)     - acknowledge the range, same as
                              push_backup_write

This way we will avoid aforementioned problems.

We can't access/uninstall fd across all processes that have it, so add kref
on the push_backup struct and tie it to number of fds. Free only when
everything is gone.

The legacy messages keep working unchanged for compatibility but we will
clean it up later.

https://virtuozzo.atlassian.net/browse/VSTOR-152912
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-push-backup.c         | 197 ++++++++++++++++++++++++----
 include/uapi/linux/dm-push-backup.h |  36 +++++
 2 files changed, 205 insertions(+), 28 deletions(-)
 create mode 100644 include/uapi/linux/dm-push-backup.h

diff --git a/drivers/md/dm-push-backup.c b/drivers/md/dm-push-backup.c
index b1861204f6e3e..1ea71cb82b9ed 100644
--- a/drivers/md/dm-push-backup.c
+++ b/drivers/md/dm-push-backup.c
@@ -15,6 +15,10 @@
 #include <linux/ctype.h>
 #include <linux/dm-io.h>
 #include <linux/blk-mq.h>
+#include <linux/anon_inodes.h>
+#include <linux/kref.h>
+#include <linux/uaccess.h>
+#include <linux/dm-push-backup.h>
 
 
 #define DM_MSG_PREFIX "push-backup"
@@ -46,6 +50,9 @@ struct push_backup {
 	u64 clu_size;
 	u64 nr_clus;
 
+	struct kref ref;
+	bool dead;
+
 	bool alive;
 	void *map;
 	u64 map_bits;
@@ -69,6 +76,16 @@ struct push_backup {
 	struct rw_semaphore ctl_rwsem;
 };
 
+static void pb_release(struct kref *ref)
+{
+	kfree(container_of(ref, struct push_backup, ref));
+}
+
+static void pb_put(struct push_backup *pb)
+{
+	kref_put(&pb->ref, pb_release);
+}
+
 static u64 pbio_first_required_for_backup_clu(struct push_backup *pb, struct pb_bio *pbio)
 {
 	u64 clu;
@@ -415,35 +432,41 @@ static int push_backup_stop(struct push_backup *pb,
 	return 0;
 }
 
-static int push_backup_read(struct push_backup *pb,
-			    char *result, unsigned int maxlen)
+static bool pb_has_event(struct push_backup *pb)
+{
+	return READ_ONCE(pb->dead) || !READ_ONCE(pb->map) ||
+	       !READ_ONCE(pb->alive) || !READ_ONCE(pb->map_bits) ||
+	       !list_empty_careful(&pb->pending);
+}
+
+/*
+ * Returns:
+ *   1       - next range to back up is stored in @clu/@nr
+ *   0       - backup is finished (nothing left to back up)
+ *   -EAGAIN - nothing pending at the moment
+ *   -ESTALE - backup is not running (not started, stopped or expired)
+ *   -ENODEV - target was destroyed
+ */
+static int pb_next_range(struct push_backup *pb, u64 *clu, u64 *nr)
 {
-	unsigned int left, right, sz = 0;
 	struct pb_bio *pbio;
+	u64 left, right;
 	int ret;
 
-	if (!pb)
-		return -EBADF;
-	if (!pb->map)
-		return -ESTALE;
-again:
-	if (wait_event_interruptible(pb->waitq,
-				     !list_empty_careful(&pb->pending) ||
-				     !pb->alive || !pb->map_bits))
-		return -EINTR;
-
 	spin_lock_irq(&pb->lock);
+	ret = -ENODEV;
+	if (pb->dead)
+		goto unlock;
 	ret = -ESTALE;
-	if (!pb->alive)
+	if (!pb->map || !pb->alive)
 		goto unlock;
 	ret = 0;
 	if (!pb->map_bits)
 		goto unlock;
+	ret = -EAGAIN;
 	pbio = list_first_entry_or_null(&pb->pending, typeof(*pbio), list);
-	if (unlikely(!pbio)) {
-		spin_unlock_irq(&pb->lock);
-		goto again;
-	}
+	if (!pbio)
+		goto unlock;
 
 	ret = -EBADMSG;
 	left = pbio_first_required_for_backup_clu(pb, pbio);
@@ -456,15 +479,47 @@ static int push_backup_read(struct push_backup *pb,
 	else
 		right = pb->nr_clus - 1;
 
-	DMEMIT("%u:%u", left, right - left + 1);
+	*clu = left;
+	*nr = right - left + 1;
 	ret = 1;
 unlock:
 	spin_unlock_irq(&pb->lock);
 	return ret;
 }
 
-static int push_backup_write(struct push_backup *pb,
-			     unsigned int clu, unsigned int nr)
+static int pb_wait_range(struct push_backup *pb, bool nonblock,
+			 u64 *clu, u64 *nr)
+{
+	int ret;
+
+	while (1) {
+		ret = pb_next_range(pb, clu, nr);
+		if (ret != -EAGAIN || nonblock)
+			return ret;
+		if (wait_event_interruptible(pb->waitq, pb_has_event(pb)))
+			return -ERESTARTSYS;
+	}
+}
+
+static int push_backup_read(struct push_backup *pb,
+			    char *result, unsigned int maxlen)
+{
+	unsigned int sz = 0;
+	u64 clu, nr;
+	int ret;
+
+	if (!pb)
+		return -EBADF;
+
+	ret = pb_wait_range(pb, false, &clu, &nr);
+	if (ret == -ERESTARTSYS)
+		ret = -EINTR;
+	if (ret == 1)
+		DMEMIT("%llu:%llu", clu, nr);
+	return ret;
+}
+
+static int push_backup_write(struct push_backup *pb, u64 clu, u64 nr)
 {
 	u64 i, key, nr_clus = pb->nr_clus;
 	LIST_HEAD(pbio_list);
@@ -475,11 +530,13 @@ static int push_backup_write(struct push_backup *pb,
 		return -EBADF;
 	if (clu >= nr_clus || nr > nr_clus - clu)
 		return -E2BIG;
-	if (!pb->map)
-		return -ESTALE;
 
 	spin_lock_irq(&pb->lock);
-	if (!pb->alive) {
+	if (pb->dead) {
+		spin_unlock_irq(&pb->lock);
+		return -ENODEV;
+	}
+	if (!pb->map || !pb->alive) {
 		spin_unlock_irq(&pb->lock);
 		return -ESTALE;
 	}
@@ -534,6 +591,73 @@ static int push_backup_write(struct push_backup *pb,
 	return 0;
 }
 
+static ssize_t pb_fd_read(struct file *file, char __user *buf,
+			  size_t count, loff_t *ppos)
+{
+	struct push_backup *pb = file->private_data;
+	struct push_backup_range range;
+	int ret;
+
+	if (count < sizeof(range))
+		return -EINVAL;
+
+	ret = pb_wait_range(pb, file->f_flags & O_NONBLOCK,
+			    &range.clu, &range.nr);
+	if (ret <= 0)
+		return ret;
+
+	if (copy_to_user(buf, &range, sizeof(range)))
+		return -EFAULT;
+	return sizeof(range);
+}
+
+static ssize_t pb_fd_write(struct file *file, const char __user *buf,
+			   size_t count, loff_t *ppos)
+{
+	struct push_backup *pb = file->private_data;
+	struct push_backup_range range;
+	int ret;
+
+	if (count != sizeof(range))
+		return -EINVAL;
+	if (copy_from_user(&range, buf, sizeof(range)))
+		return -EFAULT;
+
+	ret = push_backup_write(pb, range.clu, range.nr);
+	return ret ? : sizeof(range);
+}
+
+static int pb_fd_release(struct inode *inode, struct file *file)
+{
+	pb_put(file->private_data);
+	return 0;
+}
+
+static const struct file_operations pb_fops = {
+	.owner		= THIS_MODULE,
+	.read		= pb_fd_read,
+	.write		= pb_fd_write,
+	.release	= pb_fd_release,
+};
+
+static int push_backup_get_fd(struct push_backup *pb,
+			      char *result, unsigned int maxlen)
+{
+	unsigned int sz = 0;
+	int fd;
+
+	kref_get(&pb->ref);
+	fd = anon_inode_getfd("[push_backup]", &pb_fops, pb,
+			      O_RDWR | O_CLOEXEC);
+	if (fd < 0) {
+		pb_put(pb);
+		return fd;
+	}
+
+	DMEMIT("%d", fd);
+	return 1;
+}
+
 static int push_backup_statistics(struct push_backup *pb, char *result,
 				  unsigned int maxlen)
 {
@@ -612,6 +736,10 @@ static int pb_message(struct dm_target *ti, unsigned int argc, char **argv,
 		ret = push_backup_write(pb, val, val2);
 	} else if (!strcmp(argv[0], "push_backup_statistics")) {
 		ret = push_backup_statistics(pb, result, maxlen);
+	} else if (!strcmp(argv[0], "push_backup_get_fd")) {
+		if (argc != 1)
+			goto unlock;
+		ret = push_backup_get_fd(pb, result, maxlen);
 	} else {
 		ret = -ENOTSUPP;
 	}
@@ -626,18 +754,30 @@ static int pb_message(struct dm_target *ti, unsigned int argc, char **argv,
 }
 static void pb_destroy(struct push_backup *pb)
 {
+	void *map, *pending_map;
+
 	WARN_ON_ONCE(pb->nr_delayed);
 
+	spin_lock_irq(&pb->lock);
+	pb->dead = true;
+	map = pb->map; /* Is's not zero if stop was not called */
+	pb->map = NULL;
+	pending_map = pb->pending_map;
+	pb->pending_map = NULL;
+	spin_unlock_irq(&pb->lock);
+
+	wake_up_interruptible(&pb->waitq); /* pb->dead = true */
+
 	WARN_ON_ONCE(timer_delete_sync_try(&pb->deadline_timer) < 0);
 	if (pb->wq)
 		destroy_workqueue(pb->wq);
-	kvfree(pb->map); /* Is's not zero if stop was not called */
-	kvfree(pb->pending_map);
+	kvfree(map);
+	kvfree(pending_map);
 	kvfree(pb->pending_htable);
 	mempool_destroy(pb->pbio_pool);
 	if (pb->origin_dev)
 		dm_put_device(pb->ti, pb->origin_dev);
-	kfree(pb);
+	pb_put(pb);
 }
 
 /*
@@ -656,6 +796,7 @@ static int pb_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	pb = kzalloc(sizeof(*pb), GFP_KERNEL);
 	if (!pb)
 		goto err;
+	kref_init(&pb->ref);
 	spin_lock_init(&pb->lock);
 	init_rwsem(&pb->ctl_rwsem);
 	bio_list_init(&pb->deferred_bios);
@@ -764,7 +905,7 @@ static loff_t pb_llseek_hole(struct dm_target *ti, loff_t offset,
 
 static struct target_type pb_target = {
 	.name = "push_backup",
-	.version = {1, 0, 0},
+	.version = {1, 1, 0},
 	.features = DM_TARGET_SINGLETON,
 	.module = THIS_MODULE,
 	.ctr = pb_ctr,
diff --git a/include/uapi/linux/dm-push-backup.h b/include/uapi/linux/dm-push-backup.h
new file mode 100644
index 0000000000000..3a56f851a2d56
--- /dev/null
+++ b/include/uapi/linux/dm-push-backup.h
@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */
+/*
+ *  Copyright (c) 2026 Virtuozzo International GmbH. All rights reserved.
+ */
+
+/*
+ * Userspace interface of the dm-push-backup target's event file descriptor.
+ *
+ * The fd is obtained with the "push_backup_get_fd" target message; the fd
+ * number is returned in the message result buffer.
+ *
+ * read(fd, &range, sizeof(range)):
+ *   sizeof(range) - @range holds the next cluster range that must be backed
+ *                   up before the writes postponed on it may proceed.
+ *                   Reading does not consume anything: the same (or a wider)
+ *                   range is returned until it is acknowledged via write().
+ *   0             - backup has finished, nothing is left to back up.
+ *   -EAGAIN       - O_NONBLOCK is set and no writes are pending.
+ *   -ESTALE       - backup is not running (not started, stopped or expired).
+ *   -ENODEV       - the target has been destroyed.
+ *
+ * write(fd, &range, sizeof(range)):
+ *   acknowledge that @range has been backed up; equivalent to the
+ *   "push_backup_write clu:nr" message.
+ */
+#ifndef _UAPI_LINUX_DM_PUSH_BACKUP_H
+#define _UAPI_LINUX_DM_PUSH_BACKUP_H
+
+#include <linux/types.h>
+
+struct push_backup_range {
+	__u64 clu;	/* first cluster */
+	__u64 nr;	/* number of clusters */
+};
+
+#endif /* _UAPI_LINUX_DM_PUSH_BACKUP_H */
-- 
2.43.5

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