[PATCH RFC v6 4/5] drivers/migrate_offload: add DMA batch copy driver (dcbm)

[Shivank Garg]

Add a simple DMAEngine-based migrator that plugs into the page
migration copy-offload infrastructure and batch-copies folios via
DMA memcpy channels. It is intended for testing the offload plumbing
and as a template for future migrators (SDXI, multi-threaded CPU copy,
etc.).

On success, the dst folios are marked with FOLIO_CONTENT_COPIED so
__migrate_folio skips the per-folio copy. On any DMA error,
FOLIO_CONTENT_COPIED market is not set and the migration falls back
to per-folio CPU copy.

dcbm registers with reason mask with reason mask
MIGRATE_OFFLOAD_REASONS_ALLOWED, this set can be narrowed at runtime
via the reason_mask parameter.

Runtime controls are under /sys/module/dcbm/parameters/:

  offloading      - enable/disable DMA offload (0/1)
  nr_dma_chan     - max DMA channels to use (1..N)
  reason_mask     - reasons to offload: names (e.g. "compaction,demotion"),
		    "all"/"none" or raw numerical mask.
  folios_migrated - folios DMA-copied (write to reset)
  folios_failures - fallback count (write to reset)

CONFIG_DCBM_DMA selects MIGRATION_COPY_OFFLOAD so enabling the
driver pulls in the infrastructure automatically.

Channel acquisition uses dma_request_chan_by_mask(DMA_MEMCPY), which
works for providers that set DMA_PRIVATE (e.g. AMD PTDMA). Generic
mem-to-mem engines that do not set DMA_PRIVATE (e.g. SDXI) should
acquire channels via dma_find_channel(DMA_MEMCPY) or the async_tx
APIs, which can be added in a follow-up.

[Correctness Note:
 Karim: Descriptor can complete out of order for some dmaengines.
 so descriptor chaining may yield incorrect results. For correctness
 we need to add callback to every descriptor
 https://lore.kernel.org/all/20260619160725.lfcxrbj5go67qy6u@wrangler
 Shivank: device_prep_dma_memcpy_sg() would solve this.
]

Reviewed-by: Karim Manaouil <kmanaouil.dev@xxxxxxxxx>
Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
 drivers/Kconfig                       |   2 +
 drivers/Makefile                      |   2 +
 drivers/migrate_offload/Kconfig       |   9 +
 drivers/migrate_offload/Makefile      |   1 +
 drivers/migrate_offload/dcbm/Makefile |   1 +
 drivers/migrate_offload/dcbm/dcbm.c   | 481 ++++++++++++++++++++++++++++++++++
 6 files changed, 496 insertions(+)

diff --git a/drivers/Kconfig b/drivers/Kconfig
index f2bed2ddeb66..3e83a1475cbc 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -253,4 +253,6 @@ source "drivers/cdx/Kconfig"
 
 source "drivers/resctrl/Kconfig"
 
+source "drivers/migrate_offload/Kconfig"
+
 endmenu
diff --git a/drivers/Makefile b/drivers/Makefile
index 0841ea851847..88cb8e3e88df 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -42,6 +42,8 @@ obj-y				+= clk/
 # really early.
 obj-$(CONFIG_DMADEVICES)	+= dma/
 
+obj-$(CONFIG_MIGRATION_COPY_OFFLOAD)	+= migrate_offload/
+
 # SOC specific infrastructure drivers.
 obj-y				+= soc/
 obj-$(CONFIG_PM_GENERIC_DOMAINS)	+= pmdomain/
diff --git a/drivers/migrate_offload/Kconfig b/drivers/migrate_offload/Kconfig
new file mode 100644
index 000000000000..c9f2e21a95cf
--- /dev/null
+++ b/drivers/migrate_offload/Kconfig
@@ -0,0 +1,9 @@
+config DCBM_DMA
+	tristate "DMA Core Batch Migrator"
+	depends on MIGRATION && DMA_ENGINE && 64BIT
+	select MIGRATION_COPY_OFFLOAD
+	help
+	  DMA-based batch copy engine for page migration. Uses
+	  DMAEngine memcpy channels to offload folio data copies
+	  during migration. Primarily intended for testing the copy
+	  offload infrastructure.
diff --git a/drivers/migrate_offload/Makefile b/drivers/migrate_offload/Makefile
new file mode 100644
index 000000000000..9e16018beb15
--- /dev/null
+++ b/drivers/migrate_offload/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_DCBM_DMA)		+= dcbm/
diff --git a/drivers/migrate_offload/dcbm/Makefile b/drivers/migrate_offload/dcbm/Makefile
new file mode 100644
index 000000000000..56ba47cce0f1
--- /dev/null
+++ b/drivers/migrate_offload/dcbm/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_DCBM_DMA) += dcbm.o
diff --git a/drivers/migrate_offload/dcbm/dcbm.c b/drivers/migrate_offload/dcbm/dcbm.c
new file mode 100644
index 000000000000..bacbcc689171
--- /dev/null
+++ b/drivers/migrate_offload/dcbm/dcbm.c
@@ -0,0 +1,481 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * DMA Core Batch Migrator (DCBM)
+ *
+ * Uses DMAEngine memcpy channels to offload batch folio copies during
+ * page migration. Reference driver meant for testing the offload
+ * infrastructure.
+ *
+ * Copyright (C) 2024-26 Advanced Micro Devices, Inc.
+ */
+
+#include <linux/module.h>
+#include <linux/dma-mapping.h>
+#include <linux/dmaengine.h>
+#include <linux/migrate.h>
+#include <linux/migrate_copy_offload.h>
+
+#define MAX_DMA_CHANNELS	16
+
+static atomic_long_t folios_migrated;
+static atomic_long_t folios_failures;
+
+static bool offloading_enabled;
+static unsigned int nr_dma_channels = 1;
+static DEFINE_MUTEX(dcbm_mutex);
+
+struct dma_work {
+	struct dma_chan *chan;
+	struct completion done;
+	atomic_t pending;
+	struct sg_table *src_sgt;
+	struct sg_table *dst_sgt;
+	bool mapped;
+};
+
+static void dma_completion_callback(void *data)
+{
+	struct dma_work *work = data;
+
+	if (atomic_dec_and_test(&work->pending))
+		complete(&work->done);
+}
+
+static int setup_sg_tables(struct dma_work *work, struct list_head **src_pos,
+			   struct list_head **dst_pos, int nr)
+{
+	struct scatterlist *sg_src, *sg_dst;
+	struct device *dev;
+	int i, ret;
+
+	work->src_sgt = kmalloc_obj(*work->src_sgt, GFP_KERNEL);
+	if (!work->src_sgt)
+		return -ENOMEM;
+	work->dst_sgt = kmalloc_obj(*work->dst_sgt, GFP_KERNEL);
+	if (!work->dst_sgt) {
+		ret = -ENOMEM;
+		goto err_free_src;
+	}
+
+	ret = sg_alloc_table(work->src_sgt, nr, GFP_KERNEL);
+	if (ret)
+		goto err_free_dst;
+	ret = sg_alloc_table(work->dst_sgt, nr, GFP_KERNEL);
+	if (ret)
+		goto err_free_src_table;
+
+	sg_src = work->src_sgt->sgl;
+	sg_dst = work->dst_sgt->sgl;
+	for (i = 0; i < nr; i++) {
+		struct folio *src = list_entry(*src_pos, struct folio, lru);
+		struct folio *dst = list_entry(*dst_pos, struct folio, lru);
+
+		sg_set_folio(sg_src, src, folio_size(src), 0);
+		sg_set_folio(sg_dst, dst, folio_size(dst), 0);
+
+		*src_pos = (*src_pos)->next;
+		*dst_pos = (*dst_pos)->next;
+
+		if (i < nr - 1) {
+			sg_src = sg_next(sg_src);
+			sg_dst = sg_next(sg_dst);
+		}
+	}
+
+	dev = dmaengine_get_dma_device(work->chan);
+	if (!dev) {
+		ret = -ENODEV;
+		goto err_free_dst_table;
+	}
+	ret = dma_map_sgtable(dev, work->src_sgt, DMA_TO_DEVICE,
+			      DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
+	if (ret)
+		goto err_free_dst_table;
+	ret = dma_map_sgtable(dev, work->dst_sgt, DMA_FROM_DEVICE,
+			      DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
+	if (ret)
+		goto err_unmap_src;
+
+	/*
+	 * TODO: IOMMU may merge segments unevenly on the two sides, fall back
+	 * bail to CPU copy. In practice, I have not observed merging in tests.
+	 * Handling unequal nents is left for follow-up.
+	 */
+	if (work->src_sgt->nents != work->dst_sgt->nents) {
+		ret = -EINVAL;
+		goto err_unmap_dst;
+	}
+	work->mapped = true;
+	return 0;
+
+err_unmap_dst:
+	dma_unmap_sgtable(dev, work->dst_sgt, DMA_FROM_DEVICE,
+			  DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
+err_unmap_src:
+	dma_unmap_sgtable(dev, work->src_sgt, DMA_TO_DEVICE,
+			  DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_NO_KERNEL_MAPPING);
+err_free_dst_table:
+	sg_free_table(work->dst_sgt);
+err_free_src_table:
+	sg_free_table(work->src_sgt);
+err_free_dst:
+	kfree(work->dst_sgt);
+	work->dst_sgt = NULL;
+err_free_src:
+	kfree(work->src_sgt);
+	work->src_sgt = NULL;
+	return ret;
+}
+
+static void cleanup_dma_work(struct dma_work *works, int actual_channels)
+{
+	struct device *dev;
+	int i;
+
+	if (!works)
+		return;
+
+	for (i = 0; i < actual_channels; i++) {
+		if (!works[i].chan)
+			continue;
+
+		dev = dmaengine_get_dma_device(works[i].chan);
+
+		if (works[i].mapped)
+			dmaengine_terminate_sync(works[i].chan);
+
+		if (dev && works[i].mapped) {
+			if (works[i].src_sgt) {
+				dma_unmap_sgtable(dev, works[i].src_sgt,
+						  DMA_TO_DEVICE,
+						  DMA_ATTR_SKIP_CPU_SYNC |
+						  DMA_ATTR_NO_KERNEL_MAPPING);
+				sg_free_table(works[i].src_sgt);
+				kfree(works[i].src_sgt);
+			}
+			if (works[i].dst_sgt) {
+				dma_unmap_sgtable(dev, works[i].dst_sgt,
+						  DMA_FROM_DEVICE,
+						  DMA_ATTR_SKIP_CPU_SYNC |
+						  DMA_ATTR_NO_KERNEL_MAPPING);
+				sg_free_table(works[i].dst_sgt);
+				kfree(works[i].dst_sgt);
+			}
+		}
+		dma_release_channel(works[i].chan);
+	}
+	kfree(works);
+}
+
+static int submit_dma_transfers(struct dma_work *work)
+{
+	struct scatterlist *sg_src, *sg_dst;
+	struct dma_async_tx_descriptor *tx;
+	unsigned long flags = DMA_CTRL_ACK;
+	dma_cookie_t cookie;
+	int i;
+
+	atomic_set(&work->pending, 1);
+
+	sg_src = work->src_sgt->sgl;
+	sg_dst = work->dst_sgt->sgl;
+	for_each_sgtable_dma_sg(work->src_sgt, sg_src, i) {
+		if (i == work->src_sgt->nents - 1)
+			flags |= DMA_PREP_INTERRUPT;
+
+		tx = dmaengine_prep_dma_memcpy(work->chan,
+					       sg_dma_address(sg_dst),
+					       sg_dma_address(sg_src),
+					       sg_dma_len(sg_src), flags);
+		if (!tx) {
+			atomic_set(&work->pending, 0);
+			return -EIO;
+		}
+
+		if (i == work->src_sgt->nents - 1) {
+			tx->callback = dma_completion_callback;
+			tx->callback_param = work;
+		}
+
+		cookie = dmaengine_submit(tx);
+		if (dma_submit_error(cookie)) {
+			atomic_set(&work->pending, 0);
+			return -EIO;
+		}
+		sg_dst = sg_next(sg_dst);
+	}
+	return 0;
+}
+
+/**
+ * folios_copy_dma - copy a batch of folios via DMA memcpy
+ * @dst_list: destination folio list
+ * @src_list: source folio list
+ * @nr_folios: number of folios in each list
+ *
+ * Return: 0 on success, negative errno on failure.
+ */
+static int folios_copy_dma(struct list_head *dst_list,
+			   struct list_head *src_list, unsigned int nr_folios)
+{
+	struct folio *dst;
+	struct dma_work *works;
+	struct list_head *src_pos = src_list->next;
+	struct list_head *dst_pos = dst_list->next;
+	int i, folios_per_chan, ret;
+	dma_cap_mask_t mask;
+	int actual_channels = 0;
+	unsigned int max_channels;
+
+	max_channels = min3(READ_ONCE(nr_dma_channels), nr_folios,
+			    (unsigned int)MAX_DMA_CHANNELS);
+
+	works = kcalloc(max_channels, sizeof(*works), GFP_KERNEL);
+	if (!works)
+		return -ENOMEM;
+
+	dma_cap_zero(mask);
+	dma_cap_set(DMA_MEMCPY, mask);
+
+	for (i = 0; i < max_channels; i++) {
+		works[actual_channels].chan = dma_request_chan_by_mask(&mask);
+		if (IS_ERR(works[actual_channels].chan))
+			break;
+		init_completion(&works[actual_channels].done);
+		actual_channels++;
+	}
+
+	if (actual_channels == 0) {
+		kfree(works);
+		return -ENODEV;
+	}
+
+	for (i = 0; i < actual_channels; i++) {
+		folios_per_chan = nr_folios * (i + 1) / actual_channels -
+				(nr_folios * i) / actual_channels;
+		if (folios_per_chan == 0)
+			continue;
+
+		ret = setup_sg_tables(&works[i], &src_pos, &dst_pos,
+				      folios_per_chan);
+		if (ret)
+			goto err_cleanup;
+	}
+
+	for (i = 0; i < actual_channels; i++) {
+		if (!works[i].mapped)
+			continue;
+		ret = submit_dma_transfers(&works[i]);
+		if (ret)
+			goto err_cleanup;
+	}
+
+	for (i = 0; i < actual_channels; i++) {
+		if (atomic_read(&works[i].pending) > 0)
+			dma_async_issue_pending(works[i].chan);
+	}
+
+	for (i = 0; i < actual_channels; i++) {
+		if (atomic_read(&works[i].pending) == 0)
+			continue;
+		if (!wait_for_completion_timeout(&works[i].done,
+						 msecs_to_jiffies(10000))) {
+			ret = -ETIMEDOUT;
+			goto err_cleanup;
+		}
+	}
+
+	/*
+	 * All folios copied; mark each dst with FOLIO_CONTENT_COPIED so
+	 * __migrate_folio() skips the per-folio copy in the move phase.
+	 */
+	list_for_each_entry(dst, dst_list, lru)
+		dst->migrate_info |= FOLIO_CONTENT_COPIED;
+
+	cleanup_dma_work(works, actual_channels);
+
+	atomic_long_add(nr_folios, &folios_migrated);
+	return 0;
+
+err_cleanup:
+	pr_warn_ratelimited("dcbm: DMA copy failed (%d), falling back to CPU\n",
+			    ret);
+	cleanup_dma_work(works, actual_channels);
+
+	atomic_long_add(nr_folios, &folios_failures);
+	return ret;
+}
+
+static const struct migrator dma_migrator = {
+	.name = "DCBM",
+	.offload_copy = folios_copy_dma,
+	.owner = THIS_MODULE,
+};
+
+static unsigned long dcbm_reason_mask = MIGRATE_OFFLOAD_REASONS_ALLOWED;
+
+/* offloading: enable/disable DMA migration offload */
+static int offloading_param_set(const char *val, const struct kernel_param *kp)
+{
+	bool enable;
+	int ret;
+
+	ret = kstrtobool(val, &enable);
+	if (ret)
+		return ret;
+
+	mutex_lock(&dcbm_mutex);
+	if (enable == offloading_enabled) {
+		mutex_unlock(&dcbm_mutex);
+		return 0;
+	}
+	if (enable) {
+		ret = migrate_offload_register(&dma_migrator,
+					       READ_ONCE(dcbm_reason_mask));
+		if (ret) {
+			mutex_unlock(&dcbm_mutex);
+			return ret;
+		}
+		WRITE_ONCE(offloading_enabled, true);
+	} else {
+		migrate_offload_unregister(&dma_migrator);
+		WRITE_ONCE(offloading_enabled, false);
+	}
+	mutex_unlock(&dcbm_mutex);
+	return 0;
+}
+
+static int offloading_param_get(char *buffer, const struct kernel_param *kp)
+{
+	return sysfs_emit(buffer, "%d\n", READ_ONCE(offloading_enabled));
+}
+
+static const struct kernel_param_ops offloading_param_ops = {
+	.set = offloading_param_set,
+	.get = offloading_param_get,
+};
+module_param_cb(offloading, &offloading_param_ops, NULL, 0644);
+MODULE_PARM_DESC(offloading, "Enable DMA migration offload (0/1)");
+
+/* nr_dma_chan: max DMA channels to use per batch */
+static int nr_dma_chan_param_set(const char *val, const struct kernel_param *kp)
+{
+	unsigned int new_val;
+	int ret;
+
+	ret = kstrtouint(val, 0, &new_val);
+	if (ret)
+		return ret;
+	if (new_val < 1 || new_val > MAX_DMA_CHANNELS)
+		return -EINVAL;
+
+	mutex_lock(&dcbm_mutex);
+	WRITE_ONCE(nr_dma_channels, new_val);
+	mutex_unlock(&dcbm_mutex);
+	return 0;
+}
+
+static int nr_dma_chan_param_get(char *buffer, const struct kernel_param *kp)
+{
+	return sysfs_emit(buffer, "%u\n", READ_ONCE(nr_dma_channels));
+}
+
+static const struct kernel_param_ops nr_dma_chan_param_ops = {
+	.set = nr_dma_chan_param_set,
+	.get = nr_dma_chan_param_get,
+};
+module_param_cb(nr_dma_chan, &nr_dma_chan_param_ops, NULL, 0644);
+MODULE_PARM_DESC(nr_dma_chan, "Max DMA channels to use (1..16)");
+
+/* reason_mask: set of MR_* reasons this migrator handles */
+static int reason_mask_param_set(const char *val, const struct kernel_param *kp)
+{
+	unsigned long mask;
+	int ret;
+
+	ret = migrate_offload_reason_mask_parse(val, &mask);
+	if (ret)
+		return ret;
+
+	mutex_lock(&dcbm_mutex);
+	WRITE_ONCE(dcbm_reason_mask, mask);
+	if (offloading_enabled)
+		migrate_offload_set_reason_mask(&dma_migrator, mask);
+	mutex_unlock(&dcbm_mutex);
+	return 0;
+}
+
+static int reason_mask_param_get(char *buffer, const struct kernel_param *kp)
+{
+	return migrate_offload_reason_mask_format(buffer, READ_ONCE(dcbm_reason_mask));
+}
+
+static const struct kernel_param_ops reason_mask_param_ops = {
+	.set = reason_mask_param_set,
+	.get = reason_mask_param_get,
+};
+module_param_cb(reason_mask, &reason_mask_param_ops, NULL, 0644);
+MODULE_PARM_DESC(reason_mask,
+		 "Reasons to offload: comma-separated names (e.g. compaction,demotion), 'all', 'none', or a raw hex mask");
+
+/* folios_migrated / folios_failures: counters; any write resets to 0 */
+static int folios_migrated_param_set(const char *val, const struct kernel_param *kp)
+{
+	atomic_long_set(&folios_migrated, 0);
+	return 0;
+}
+
+static int folios_migrated_param_get(char *buffer, const struct kernel_param *kp)
+{
+	return sysfs_emit(buffer, "%ld\n", atomic_long_read(&folios_migrated));
+}
+
+static const struct kernel_param_ops folios_migrated_param_ops = {
+	.set = folios_migrated_param_set,
+	.get = folios_migrated_param_get,
+};
+module_param_cb(folios_migrated, &folios_migrated_param_ops, NULL, 0644);
+MODULE_PARM_DESC(folios_migrated, "Folios DMA-copied (write to reset)");
+
+static int folios_failures_param_set(const char *val, const struct kernel_param *kp)
+{
+	atomic_long_set(&folios_failures, 0);
+	return 0;
+}
+
+static int folios_failures_param_get(char *buffer, const struct kernel_param *kp)
+{
+	return sysfs_emit(buffer, "%ld\n", atomic_long_read(&folios_failures));
+}
+
+static const struct kernel_param_ops folios_failures_param_ops = {
+	.set = folios_failures_param_set,
+	.get = folios_failures_param_get,
+};
+module_param_cb(folios_failures, &folios_failures_param_ops, NULL, 0644);
+MODULE_PARM_DESC(folios_failures, "DMA-copy failure count (write to reset)");
+
+static int __init dcbm_init(void)
+{
+	pr_info("dcbm: DMA Core Batch Migrator initialized\n");
+	return 0;
+}
+
+static void __exit dcbm_exit(void)
+{
+	mutex_lock(&dcbm_mutex);
+	if (offloading_enabled) {
+		migrate_offload_unregister(&dma_migrator);
+		offloading_enabled = false;
+	}
+	mutex_unlock(&dcbm_mutex);
+
+	pr_info("dcbm: DMA Core Batch Migrator unloaded\n");
+}
+
+module_init(dcbm_init);
+module_exit(dcbm_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Shivank Garg");
+MODULE_DESCRIPTION("DMA Core Batch Migrator");

-- 
2.43.0