Re: [RFC PATCH v3 3/8] mm: Hot page tracking and promotion

[Alok Rathore]

On 10/11/25 10:53AM, Bharata B Rao wrote:
> This introduces a sub-system for collecting memory access
> information from different sources. It maintains the hotness
> information based on the access history and time of access.
>
> Additionally, it provides per-lowertier-node kernel threads
> (named kmigrated) that periodically promote the pages that
> are eligible for promotion.
>
> Sub-systems that generate hot page access info can report that
> using this API:
>
> int pghot_record_access(unsigned long pfn, int nid, int src,
>                        unsigned long time)
>
> @pfn: The PFN of the memory accessed
> @nid: The accessing NUMA node ID
> @src: The temperature source (sub-system) that generated the
>      access info
> @time: The access time in jiffies
>
> Some temperature sources may not provide the nid from which
> the page was accessed. This is true for sources that use
> page table scanning for PTE Accessed bit. For such sources,
> the default toptier node to which such pages should be promoted
> is hard coded.
>
> Also, the access time provided some sources may at best be
> considered approximate. This is especially true for hot pages
> detected by PTE A bit scanning.
>
> The hotness information is stored for every page of lower
> tier memory in an unsigned long variable that is part of
> mem_section data structure.
>
> kmigrated is a per-lowertier-node kernel thread that migrates
> the folios marked for migration in batches. Each kmigrated
> thread walks the PFN range spanning its node and checks
> for potential migration candidates.
>
> Signed-off-by: Bharata B Rao <bharata@xxxxxxx>
> ---
> include/linux/mmzone.h        |  14 ++
> include/linux/pghot.h         |  52 ++++
> include/linux/vm_event_item.h |   4 +
> mm/Kconfig                    |  11 +
> mm/Makefile                   |   1 +
> mm/mm_init.c                  |  10 +
> mm/page_ext.c                 |  11 +
> mm/pghot.c                    | 446 ++++++++++++++++++++++++++++++++++
> mm/vmstat.c                   |   4 +
> 9 files changed, 553 insertions(+)
> create mode 100644 include/linux/pghot.h
> create mode 100644 mm/pghot.c
>
> +

<snip>

> +/*
> + * Walks the PFNs of the zone, isolates and migrates them in batches.
> + */
> +static void kmigrated_walk_zone(unsigned long start_pfn, unsigned long end_pfn,
> +				int src_nid)
> +{
> +	int cur_nid = NUMA_NO_NODE;
> +	LIST_HEAD(migrate_list);
> +	int batch_count = 0;
> +	struct folio *folio;
> +	struct page *page;
> +	unsigned long pfn;
> +
> +	pfn = start_pfn;
> +	do {
> +		unsigned long nid = NUMA_NO_NODE, freq = 0, time = 0, nr = 1;
> +
> +		if (!pfn_valid(pfn))
> +			goto out_next;
> +
> +		page = pfn_to_online_page(pfn);
> +		if (!page)
> +			goto out_next;
> +
> +		folio = page_folio(page);
> +		nr = folio_nr_pages(folio);
> +		if (folio_nid(folio) != src_nid)
> +			goto out_next;
> +
> +		if (!folio_test_lru(folio))
> +			goto out_next;
> +
> +		if (pghot_get_hotness(pfn, &nid, &freq, &time))

Better to remove freq value, it’s not used later.

Regards,
Alok Rathore