Re: [RFC V3 7/9] dcbm: add dma core batch migrator for batch page offloading

[Jonathan Cameron]

On Tue, 23 Sep 2025 17:47:42 +0000
Shivank Garg <shivankg@xxxxxxx> wrote:

> The dcbm (DMA core batch migrator) provides a generic interface using
> DMAEngine for end-to-end testing of the batch page migration offload
> feature.
> 
> Enable DCBM offload:
> echo 1 > /sys/kernel/dcbm/offloading
> echo NR_DMA_CHAN_TO_USE > /sys/kernel/dcbm/nr_dma_chan
> 
> Disable DCBM offload:
> echo 0 > /sys/kernel/dcbm/offloading
> 
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
Hi Shivank,

Some minor comments inline.

J
> ---
>  drivers/migoffcopy/Kconfig       |   8 +
>  drivers/migoffcopy/Makefile      |   1 +
>  drivers/migoffcopy/dcbm/Makefile |   1 +
>  drivers/migoffcopy/dcbm/dcbm.c   | 415 +++++++++++++++++++++++++++++++
>  4 files changed, 425 insertions(+)
>  create mode 100644 drivers/migoffcopy/dcbm/Makefile
>  create mode 100644 drivers/migoffcopy/dcbm/dcbm.c
> 
> diff --git a/drivers/migoffcopy/Kconfig b/drivers/migoffcopy/Kconfig
> index e73698af3e72..c1b2eff7650d 100644
> --- a/drivers/migoffcopy/Kconfig
> +++ b/drivers/migoffcopy/Kconfig
> @@ -6,4 +6,12 @@ config MTCOPY_CPU
>           Interface MT COPY CPU driver for batch page migration
>           offloading. Say Y if you want to try offloading with
>           MultiThreaded CPU copy APIs.

I'd put a blank line here.

> +config DCBM_DMA
> +       bool "DMA Core Batch Migrator"
> +       depends on OFFC_MIGRATION && DMA_ENGINE
> +       default n

no need to say this. Everything is default n.

> +       help
> +         Interface DMA driver for batch page migration offloading.
> +         Say Y if you want to try offloading with DMAEngine APIs
> +         based driver.

Similar comment on the 'try'

>  
> diff --git a/drivers/migoffcopy/Makefile b/drivers/migoffcopy/Makefile
> index 0a3c356d67e6..dedc86ff54c1 100644
> --- a/drivers/migoffcopy/Makefile
> +++ b/drivers/migoffcopy/Makefile
> @@ -1 +1,2 @@
>  obj-$(CONFIG_MTCOPY_CPU) += mtcopy/
> +obj-$(CONFIG_DCBM_DMA)		+= dcbm/
> diff --git a/drivers/migoffcopy/dcbm/Makefile b/drivers/migoffcopy/dcbm/Makefile
> new file mode 100644
> index 000000000000..56ba47cce0f1
> --- /dev/null
> +++ b/drivers/migoffcopy/dcbm/Makefile
> @@ -0,0 +1 @@
> +obj-$(CONFIG_DCBM_DMA) += dcbm.o
> diff --git a/drivers/migoffcopy/dcbm/dcbm.c b/drivers/migoffcopy/dcbm/dcbm.c
> new file mode 100644
> index 000000000000..87a58c0c3b9b
> --- /dev/null
> +++ b/drivers/migoffcopy/dcbm/dcbm.c


> +/**
> + * folios_copy_dma - Copy folios using DMA engine
> + * @dst_list: Destination folio list
> + * @src_list: Source folio list
> + * @nr_folios: Number of folios to copy
> + *
> + * Return: 0. Fallback to CPU copy on any error.
> + */
> +static int folios_copy_dma(struct list_head *dst_list,
> +			   struct list_head *src_list,
> +			   unsigned int nr_folios)
> +{
> +	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 = 0;
> +	dma_cap_mask_t mask;
> +	int actual_channels = 0;
> +	int max_channels;
> +
> +	max_channels = min3(nr_dma_channels, nr_folios, MAX_DMA_CHANNELS);
> +
> +	works = kcalloc(max_channels, sizeof(*works), GFP_KERNEL);
> +	if (!works)
> +		goto fallback;
> +
> +	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);
> +		goto fallback;
> +	}
> +
> +	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 cleanup;
> +	}

Indent issues here.

> +
> +	for (i = 0; i < actual_channels; i++) {
> +		ret = submit_dma_transfers(&works[i]);
> +		if (ret) {
> +			dev_err(dmaengine_get_dma_device(works[i].chan),
> +				"Failed to submit transfers for channel %d\n", i);
> +			goto 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) {
I'd flip logic to something like.
		if (!atomic_read(&works[i].pending)
			continue;

		if (!wait_for_...

Just to reduce the deep indent.

> +			if (!wait_for_completion_timeout(&works[i].done, msecs_to_jiffies(10000))) {
> +				dev_err(dmaengine_get_dma_device(works[i].chan),
> +					"DMA timeout on channel %d\n", i);
> +				ret = -ETIMEDOUT;
> +				goto cleanup;
> +			}
> +		}
> +	}
> +
> +cleanup:
> +	cleanup_dma_work(works, actual_channels);
> +	if (ret)
> +		goto fallback;

This goto goto dance is probably not worth it.  I'd just duplicate the
cleanup_dma_work() call to have a copy in the error path and one in the non error
path.  Then you just end up with a conventional error block of labels + stuff to do.

> +	return 0;
> +fallback:
> +	/* Fall back to CPU copy */
> +	pr_err("DCBM: Falling back to CPU copy\n");
> +	folios_mc_copy(dst_list, src_list, nr_folios);
> +	return 0;
> +}

> +static ssize_t offloading_store(struct kobject *kobj,
> +				struct kobj_attribute *attr,
> +				const char *buf, size_t count)
> +{
> +	bool enable;
> +	int ret;
> +
> +	ret = kstrtobool(buf, &enable);
> +	if (ret)
> +		return ret;
> +
> +	mutex_lock(&dcbm_mutex);
> +
> +	if (enable == offloading_enabled) {
> +		pr_err("migration offloading is already %s\n",
> +			 enable ? "ON" : "OFF");

To me that's not an error. Pointless, but not worth moaning about.
Just exit saying nothing.

> +		goto out;
> +	}
> +
> +	if (enable) {
> +		start_offloading(&dma_migrator);
> +		offloading_enabled = true;
> +		pr_info("migration offloading is now ON\n");
> +	} else {
> +		stop_offloading();
> +		offloading_enabled = false;
> +		pr_info("migration offloading is now OFF\n");
> +	}
> +out:
> +	mutex_unlock(&dcbm_mutex);

Perhaps guard and direct return above


> +	return count;
> +}

> +static struct kobj_attribute offloading_attr = __ATTR_RW(offloading);
> +static struct kobj_attribute nr_dma_chan_attr = __ATTR_RW(nr_dma_chan);
> +
> +static struct attribute *dcbm_attrs[] = {
> +	&offloading_attr.attr,
> +	&nr_dma_chan_attr.attr,
> +	NULL,
Trivial but doesn't need a trailing comma given this is a terminating entry
and nothing should ever come after it.

> +};
> +ATTRIBUTE_GROUPS(dcbm);
> +
> +static struct kobject *dcbm_kobj;
> +
> +static int __init dcbm_init(void)
> +{
> +	int ret;
> +
> +	dcbm_kobj = kobject_create_and_add("dcbm", kernel_kobj);
> +	if (!dcbm_kobj)
> +		return -ENOMEM;
> +
> +	ret = sysfs_create_groups(dcbm_kobj, dcbm_groups);
Why use a group here and separate files in the CPU thread one?

I'd prefer a group there as well given slightly simpler error
handling as seen here.


> +	if (ret) {
> +		kobject_put(dcbm_kobj);
> +		return ret;
> +	}
> +
> +	pr_info("DMA Core Batch Migrator initialized\n");
> +	return 0;
> +}
> +