Re: [PATCH 5/7] mm/migrate: add copy offload registration infrastructure

[David Hildenbrand (Arm)]

On 4/28/26 17:50, Shivank Garg wrote:
> Add a registration interface that lets a single offload provider
> (DMA, multi-threaded CPU copy, etc) take over the batch folio copy
> performed by migrate_pages_batch().
> 
> The provider fills in a struct migrator with an offload_copy()
> callback and calls migrate_offload_register(). Registration patches
> the migrate_offload_copy() static_call and flips the
> migrate_offload_enabled static branch. The migrate_offload_unregister()
> reverts both.
> 
> Whether a migration reason is batch-copy eligible is decided by the
> core in migrate_offload_do_batch(). A migrator may decline a particular
> batch (e.g. when nr_batch is too small to amortize setup) by returning
> -EOPNOTSUPP, and the move phase falls back to per-folio CPU copy.
> 
> Only one migrator can be active at a time. A second registration
> returns -EBUSY, and only the active migrator can unregister itself.
> The static_call dispatch is protected by SRCU so that the
> synchronize_srcu() in unregister waits for all in-flight copy before
> the module reference is dropped.
> 
> Co-developed-by: Mike Day <michael.day@xxxxxxx>
> Signed-off-by: Mike Day <michael.day@xxxxxxx>
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> ---
>  include/linux/migrate_copy_offload.h | 44 +++++++++++++
>  mm/Kconfig                           |  6 ++
>  mm/Makefile                          |  1 +
>  mm/migrate.c                         | 57 +++++++++++++++--
>  mm/migrate_copy_offload.c            | 94 ++++++++++++++++++++++++++++
>  5 files changed, 198 insertions(+), 4 deletions(-)
>  create mode 100644 include/linux/migrate_copy_offload.h
>  create mode 100644 mm/migrate_copy_offload.c
> 
> diff --git a/include/linux/migrate_copy_offload.h b/include/linux/migrate_copy_offload.h
> new file mode 100644
> index 000000000000..d68b10a84743
> --- /dev/null
> +++ b/include/linux/migrate_copy_offload.h
> @@ -0,0 +1,44 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_MIGRATE_COPY_OFFLOAD_H
> +#define _LINUX_MIGRATE_COPY_OFFLOAD_H
> +
> +#include <linux/errno.h>
> +#include <linux/jump_label.h>
> +#include <linux/srcu.h>
> +#include <linux/types.h>
> +
> +struct list_head;
> +struct module;
> +
> +#define MIGRATOR_NAME_LEN 32
> +
> +/**
> + * struct migrator - batch-copy provider for page migration.
> + * @name: name of the provider.
> + * @offload_copy: copy @folio_cnt folios from @src_list to @dst_list.
> + *
> + *	The migrator may inspect @folio_cnt to decide whether the batch
> + * 	is worth offloading, e.g. skip when the batch is too small to
> + * 	amortize setup cost. If returns error, the core falls back to CPU copy.
> + *
> + * @owner: module providing the migrator.
> + */
> +struct migrator {
> +	char name[MIGRATOR_NAME_LEN];
> +	int (*offload_copy)(struct list_head *dst_list,
> +			    struct list_head *src_list,
> +			    unsigned int folio_cnt);
> +	struct module *owner;
> +};
> +
> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
> +extern struct static_key_false migrate_offload_enabled;
> +extern struct srcu_struct migrate_offload_srcu;
> +int migrate_offload_register(struct migrator *m);
> +int migrate_offload_unregister(struct migrator *m);
> +#else
> +static inline int migrate_offload_register(struct migrator *m) { return -EOPNOTSUPP; }
> +static inline int migrate_offload_unregister(struct migrator *m) { return -EOPNOTSUPP; }
> +#endif
> +
> +#endif /* _LINUX_MIGRATE_COPY_OFFLOAD_H */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e8bf1e9e6ad9..325d79619680 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -647,6 +647,12 @@ config MIGRATION
>  config DEVICE_MIGRATION
>  	def_bool MIGRATION && ZONE_DEVICE
>  
> +# Page-migration batch-copy offload infrastructure.
> +# Selected by migrator drivers (e.g. CONFIG_DCBM_DMA).
> +config MIGRATION_COPY_OFFLOAD
> +	bool
> +	depends on MIGRATION
> +
>  config ARCH_ENABLE_HUGEPAGE_MIGRATION
>  	bool
>  
> diff --git a/mm/Makefile b/mm/Makefile
> index 8ad2ab08244e..db1ac8097089 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -96,6 +96,7 @@ obj-$(CONFIG_FAILSLAB) += failslab.o
>  obj-$(CONFIG_FAIL_PAGE_ALLOC) += fail_page_alloc.o
>  obj-$(CONFIG_MEMTEST)		+= memtest.o
>  obj-$(CONFIG_MIGRATION) += migrate.o
> +obj-$(CONFIG_MIGRATION_COPY_OFFLOAD) += migrate_copy_offload.o
>  obj-$(CONFIG_NUMA) += memory-tiers.o
>  obj-$(CONFIG_DEVICE_MIGRATION) += migrate_device.o
>  obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6c2f1cb66f96..9af070f9a1f2 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -44,6 +44,8 @@
>  #include <linux/memory-tiers.h>
>  #include <linux/pagewalk.h>
>  #include <linux/jump_label.h>
> +#include <linux/static_call.h>
> +#include <linux/migrate_copy_offload.h>
>  
>  #include <asm/tlbflush.h>
>  
> @@ -54,6 +56,51 @@
>  
>  DEFINE_STATIC_KEY_FALSE(migrate_offload_enabled);
>  
> +#ifdef CONFIG_MIGRATION_COPY_OFFLOAD
> +DEFINE_SRCU(migrate_offload_srcu);
> +DEFINE_STATIC_CALL(migrate_offload_copy, folios_mc_copy);
> +
> +static bool migrate_offload_do_batch(int reason)
> +{
> +	if (!static_branch_unlikely(&migrate_offload_enabled))
> +		return false;
> +
> +	switch (reason) {
> +	case MR_COMPACTION:
> +	case MR_SYSCALL:
> +	case MR_DEMOTION:
> +	case MR_NUMA_MISPLACED:
> +		return true;
> +	default:
> +		return false;


What's the exact reason we don't do this for hotunplug etc? IOW, why do we make
this depend on a reason?


-- 
Cheers,

David