Re: [RFC PATCH v6 2/5] mm: migrate: Add migrate_misplaced_folios_batch()

[Donet Tom <donettom@linux.ibm.com>]

Hi Bharata

On 3/23/26 3:21 PM, Bharata B Rao wrote:
> From: Gregory Price <gourry@gourry.net>
>
> Tiered memory systems often require migrating multiple folios at once.
> Currently, migrate_misplaced_folio() handles only one folio per call,
> which is inefficient for batch operations. This patch introduces
> migrate_misplaced_folios_batch(), a batch variant that leverages
> migrate_pages() internally for improved performance.
>
> The caller must isolate folios beforehand using
> migrate_misplaced_folio_prepare(). On return, the folio list will be
> empty regardless of success or failure.
>
> This function will be used by pghot kmigrated thread.
>
> Signed-off-by: Gregory Price <gourry@gourry.net>
> [Rewrote commit description]
> Signed-off-by: Bharata B Rao <bharata@amd.com>
> ---
>   include/linux/migrate.h |  6 ++++++
>   mm/migrate.c            | 48 +++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 54 insertions(+)
>
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index d5af2b7f577b..5c1e2691cec2 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -111,6 +111,7 @@ static inline void softleaf_entry_wait_on_locked(softleaf_t entry, spinlock_t *p
>   int migrate_misplaced_folio_prepare(struct folio *folio,
>   		struct vm_area_struct *vma, int node);
>   int migrate_misplaced_folio(struct folio *folio, int node);
> +int migrate_misplaced_folios_batch(struct list_head *folio_list, int node);
>   #else
>   static inline int migrate_misplaced_folio_prepare(struct folio *folio,
>   		struct vm_area_struct *vma, int node)
> @@ -121,6 +122,11 @@ static inline int migrate_misplaced_folio(struct folio *folio, int node)
>   {
>   	return -EAGAIN; /* can't migrate now */
>   }
> +static inline int migrate_misplaced_folios_batch(struct list_head *folio_list,
> +						 int node)
> +{
> +	return -EAGAIN; /* can't migrate now */
> +}
>   #endif /* CONFIG_NUMA_BALANCING */
>   
>   #ifdef CONFIG_MIGRATION
> diff --git a/mm/migrate.c b/mm/migrate.c
> index a15184950e65..94daec0f49ef 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2751,5 +2751,53 @@ int migrate_misplaced_folio(struct folio *folio, int node)
>   	BUG_ON(!list_empty(&migratepages));
>   	return nr_remaining ? -EAGAIN : 0;
>   }
> +
> +/**
> + * migrate_misplaced_folios_batch() - Batch variant of migrate_misplaced_folio
> + * Attempts to migrate a folio list to the specified destination.
> + * @folio_list: Isolated list of folios to be batch-migrated.
> + * @node: The NUMA node ID to where the folios should be migrated.
> + *
> + * Caller is expected to have isolated the folios by calling
> + * migrate_misplaced_folio_prepare(), which will result in an
> + * elevated reference count on the folio. All the isolated folios
> + * in the list must belong to the same memcg so that NUMA_PAGE_MIGRATE
> + * stat can be attributed correctly to the memcg.
> + *
> + * This function will un-isolate the folios, drop the elevated reference
> + * and remove them from the list before returning. This is called
> + * only for batched promotion of hot pages from lower tier nodes.
> + *
> + * Return: 0 on success and -EAGAIN on failure or partial migration.
> + *         On return, @folio_list will be empty regardless of success/failure.
> + */
> +int migrate_misplaced_folios_batch(struct list_head *folio_list, int node)
> +{
> +	pg_data_t *pgdat = NODE_DATA(node);
> +	struct mem_cgroup *memcg = NULL;
> +	unsigned int nr_succeeded = 0;
> +	int nr_remaining;
> +
> +	if (!list_empty(folio_list)) {
>
We seem to proceed even when the list is empty. Should we instead return 
early in that case?

> +		struct folio *first = list_first_entry(folio_list, struct folio, lru);
> +		memcg = get_mem_cgroup_from_folio(first);


I had a small question—are we ensuring that a single list contains 
folios from the same memcg?


-Donet

> +	}
> +
> +	nr_remaining = migrate_pages(folio_list, alloc_misplaced_dst_folio,
> +				     NULL, node, MIGRATE_ASYNC,
> +				     MR_NUMA_MISPLACED, &nr_succeeded);
> +	if (nr_remaining)
> +		putback_movable_pages(folio_list);
> +
> +	if (nr_succeeded) {
> +		count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_succeeded);
> +		mod_node_page_state(pgdat, PGPROMOTE_SUCCESS, nr_succeeded);
> +		count_memcg_events(memcg, NUMA_PAGE_MIGRATE, nr_succeeded);
> +	}
> +
> +	mem_cgroup_put(memcg);
> +	WARN_ON(!list_empty(folio_list));
> +	return nr_remaining ? -EAGAIN : 0;
> +}
>   #endif /* CONFIG_NUMA_BALANCING */
>   #endif /* CONFIG_NUMA */