[PATCH v6 06/10] mm/memory_hotplug: add offline_and_remove_memory_ranges()

[Gregory Price <gourry@gourry.net>]

offline_and_remove_memory() handles a single contiguous range.

Callers that manage a device composed of several ranges (dax/kmem)
currently have to call it in a loop, which gives up atomicity.

In addition to pushing rollback logic into the driver, the lack
of atomicity creates a race condition between system daemons trying
to manage the same resource:

   - Manager 1:  Offlines memory blocks.    Removes device.
                                        ^^^^
   - Manager 2:  Detects offline memory blocks, re-onlines them.

Add offline_and_remove_memory_ranges(), which takes an array of ranges
and processes them as one operation under a single lock_device_hotplug():

  - Phase 1 offlines every block of every range.
  - Phase 2 removes the ranges only if all ranges are offline.
  - If any offline fails, the whole operation is reverted.

This gives callers all-or-nothing semantics for the offline step, so a
failed or interrupted unplug leaves the device in a consistent state.

This also resolves the battling managers race - the second manager's
operation simply fails when the block is destroyed / cannot be onlined.

offline_and_remove_memory() becomes a thin wrapper that passes its single
range to the new helper, so the offline/rollback logic lives in one place.

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Gregory Price <gourry@gourry.net>
---
 include/linux/memory_hotplug.h |  8 +++
 mm/memory_hotplug.c            | 93 ++++++++++++++++++++++++----------
 2 files changed, 73 insertions(+), 28 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index ff3b865ea7e7..db10d50f30ae 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -268,6 +268,8 @@ extern int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 extern int remove_memory(u64 start, u64 size);
 extern void __remove_memory(u64 start, u64 size);
 extern int offline_and_remove_memory(u64 start, u64 size);
+int offline_and_remove_memory_ranges(const struct range *ranges,
+		unsigned int nr_ranges);
 
 #else
 static inline void try_offline_node(int nid) {}
@@ -284,6 +286,12 @@ static inline int remove_memory(u64 start, u64 size)
 }
 
 static inline void __remove_memory(u64 start, u64 size) {}
+
+static inline int offline_and_remove_memory_ranges(const struct range *ranges,
+		unsigned int nr_ranges)
+{
+	return -EBUSY;
+}
 #endif /* CONFIG_MEMORY_HOTREMOVE */
 
 #ifdef CONFIG_MEMORY_HOTPLUG
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a66346def504..3225364bec2f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -2429,58 +2429,95 @@ static int try_reonline_memory_block(struct memory_block *mem, void *arg)
  */
 int offline_and_remove_memory(u64 start, u64 size)
 {
-	const unsigned long mb_count = size / memory_block_size_bytes();
+	struct range range = {
+		.start = start,
+		.end = start + size - 1,
+	};
+
+	return offline_and_remove_memory_ranges(&range, 1);
+}
+EXPORT_SYMBOL_GPL(offline_and_remove_memory);
+
+/**
+ * offline_and_remove_memory_ranges - offline and remove multiple memory ranges
+ * @ranges: array of physical address ranges to offline and remove
+ * @nr_ranges: number of entries in @ranges
+ *
+ * Offline and remove several memory ranges as one operation, serialized
+ * against other hotplug operations by a single lock_device_hotplug().
+ *
+ * This offlines all ranges before removing any of them.  If offlining any
+ * range fails, the entire process is reverted and nothing is removed.
+ * This provides a fully atomic semantic for unplugging an entire device.
+ *
+ * Each range must be memory-block aligned in start and size.
+ *
+ * Return: 0 on success, negative errno otherwise.  On failure no range has
+ * been removed.
+ */
+int offline_and_remove_memory_ranges(const struct range *ranges,
+		unsigned int nr_ranges)
+{
+	unsigned long mb_count = 0;
 	uint8_t *online_types, *tmp;
-	int rc;
+	unsigned int i;
+	int rc = 0;
 
-	if (!IS_ALIGNED(start, memory_block_size_bytes()) ||
-	    !IS_ALIGNED(size, memory_block_size_bytes()) || !size)
+	if (!ranges || !nr_ranges)
 		return -EINVAL;
 
+	for (i = 0; i < nr_ranges; i++) {
+		const u64 start = ranges[i].start;
+		const u64 size = range_len(&ranges[i]);
+
+		if (!IS_ALIGNED(start, memory_block_size_bytes()) ||
+		    !IS_ALIGNED(size, memory_block_size_bytes()) || !size)
+			return -EINVAL;
+		mb_count += size / memory_block_size_bytes();
+	}
+
 	/*
-	 * We'll remember the old online type of each memory block, so we can
-	 * try to revert whatever we did when offlining one memory block fails
-	 * after offlining some others succeeded.
+	 * Remember the old online type of every memory block across all ranges,
+	 * so we can revert if offlining a later block fails.  All entries start
+	 * as MMOP_OFFLINE so blocks we never touched are skipped on rollback.
 	 */
 	online_types = kmalloc_array(mb_count, sizeof(*online_types),
 				     GFP_KERNEL);
 	if (!online_types)
 		return -ENOMEM;
-	/*
-	 * Initialize all states to MMOP_OFFLINE, so when we abort processing in
-	 * try_offline_memory_block(), we'll skip all unprocessed blocks in
-	 * try_reonline_memory_block().
-	 */
 	memset(online_types, MMOP_OFFLINE, mb_count);
 
 	lock_device_hotplug();
 
+	/* Phase 1: offline every block in every range. */
 	tmp = online_types;
-	rc = walk_memory_blocks(start, size, &tmp, try_offline_memory_block);
-
-	/*
-	 * In case we succeeded to offline all memory, remove it.
-	 * This cannot fail as it cannot get onlined in the meantime.
-	 */
-	if (!rc) {
-		rc = try_remove_memory(start, size);
+	for (i = 0; i < nr_ranges; i++) {
+		rc = walk_memory_blocks(ranges[i].start, range_len(&ranges[i]),
+					&tmp, try_offline_memory_block);
 		if (rc)
-			pr_err("%s: Failed to remove memory: %d", __func__, rc);
+			break;
 	}
 
-	/*
-	 * Rollback what we did. While memory onlining might theoretically fail
-	 * (nacked by a notifier), it barely ever happens.
-	 */
+	/* If any failure occurred at all, rollback any changes and bail */
 	if (rc) {
 		tmp = online_types;
-		walk_memory_blocks(start, size, &tmp,
-				   try_reonline_memory_block);
+		for (i = 0; i < nr_ranges; i++)
+			walk_memory_blocks(ranges[i].start,
+					   range_len(&ranges[i]), &tmp,
+					   try_reonline_memory_block);
+		goto out_unlock;
 	}
+
+	/* Phase 2: Remove. This should never fail holding the hotplug lock */
+	for (i = 0; i < nr_ranges; i++)
+		WARN_ON_ONCE(try_remove_memory(ranges[i].start,
+					       range_len(&ranges[i])));
+
+out_unlock:
 	unlock_device_hotplug();
 
 	kfree(online_types);
 	return rc;
 }
-EXPORT_SYMBOL_GPL(offline_and_remove_memory);
+EXPORT_SYMBOL_GPL(offline_and_remove_memory_ranges);
 #endif /* CONFIG_MEMORY_HOTREMOVE */
-- 
2.53.0-Meta