Re: [PATCH 1/4] mm: Move demotion related functions in memory-tiers.c

Alexandre Ghiti
2026-03-13T13:27:56Z

Hi Tom,

On 3/12/26 09:44, Donet Tom wrote:
>
> Hi Alexander
>
> On 3/11/26 4:32 PM, Alexandre Ghiti wrote:
>> Let's have all the demotion functions in this file, no functional
>> change intended.
>>
>> Suggested-by: Gregory Price <gourry@gourry.net>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>   include/linux/memory-tiers.h | 18 ++++++++
>>   mm/memory-tiers.c            | 75 +++++++++++++++++++++++++++++++++
>>   mm/vmscan.c                  | 80 +-----------------------------------
>>   3 files changed, 94 insertions(+), 79 deletions(-)
>>
>> diff --git a/include/linux/memory-tiers.h b/include/linux/memory-tiers.h
>> index 96987d9d95a8..0bf0d002939e 100644
>> --- a/include/linux/memory-tiers.h
>> +++ b/include/linux/memory-tiers.h
>> @@ -56,6 +56,9 @@ void mt_put_memory_types(struct list_head
>> *memory_types);
>>   int next_demotion_node(int node, const nodemask_t *allowed_mask);
>>   void node_get_allowed_targets(pg_data_t *pgdat, nodemask_t *targets);
>>   bool node_is_toptier(int node);
>> +unsigned int mt_demote_folios(struct list_head *demote_folios,
>> +                  struct pglist_data *pgdat,
>> +                  struct mem_cgroup *memcg);
>>   #else
>>   static inline int next_demotion_node(int node, const nodemask_t
>> *allowed_mask)
>>   {
>> @@ -71,6 +74,14 @@ static inline bool node_is_toptier(int node)
>>   {
>>       return true;
>>   }
>> +
>> +static inline unsigned int mt_demote_folios(struct list_head
>> *demote_folios,
>> +                        struct pglist_data *pgdat,
>> +                        struct mem_cgroup *memcg)
>> +{
>> +    return 0;
>> +}
>> +
>>   #endif
>>     #else
>> @@ -116,6 +127,13 @@ static inline bool node_is_toptier(int node)
>>       return true;
>>   }
>>   +static inline unsigned int mt_demote_folios(struct list_head
>> *demote_folios,
>> +                        struct pglist_data *pgdat,
>> +                        struct mem_cgroup *memcg)
>> +{
>> +    return 0;
>> +}
>> +
>>   static inline int register_mt_adistance_algorithm(struct
>> notifier_block *nb)
>>   {
>>       return 0;
>> diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
>> index 986f809376eb..afdf21738a54 100644
>> --- a/mm/memory-tiers.c
>> +++ b/mm/memory-tiers.c
>> @@ -7,6 +7,7 @@
>>   #include <linux/memory-tiers.h>
>>   #include <linux/notifier.h>
>>   #include <linux/sched/sysctl.h>
>> +#include <linux/migrate.h>
>>     #include "internal.h"
>>   @@ -373,6 +374,80 @@ int next_demotion_node(int node, const
>> nodemask_t *allowed_mask)
>>       return find_next_best_node(node, &mask);
>>   }
>>   +static struct folio *alloc_demote_folio(struct folio *src,
>> +                    unsigned long private)
>> +{
>> +    struct folio *dst;
>> +    nodemask_t *allowed_mask;
>> +    struct migration_target_control *mtc;
>> +
>> +    mtc = (struct migration_target_control *)private;
>> +
>> +    allowed_mask = mtc->nmask;
>> +    /*
>> +     * make sure we allocate from the target node first also trying to
>> +     * demote or reclaim pages from the target node via kswapd if we
>> are
>> +     * low on free memory on target node. If we don't do this and if
>> +     * we have free memory on the slower(lower) memtier, we would start
>> +     * allocating pages from slower(lower) memory tiers without even
>> forcing
>> +     * a demotion of cold pages from the target memtier. This can
>> result
>> +     * in the kernel placing hot pages in slower(lower) memory tiers.
>> +     */
>> +    mtc->nmask = NULL;
>> +    mtc->gfp_mask |= __GFP_THISNODE;
>> +    dst = alloc_migration_target(src, (unsigned long)mtc);
>> +    if (dst)
>> +        return dst;
>> +
>> +    mtc->gfp_mask &= ~__GFP_THISNODE;
>> +    mtc->nmask = allowed_mask;
>> +
>> +    return alloc_migration_target(src, (unsigned long)mtc);
>> +}
>> +
>> +unsigned int mt_demote_folios(struct list_head *demote_folios,
>
>
> Demotion will happen only when different memory tiers are present,
> right? Since demote_folios() already implies that the folios are being
> demoted to a lower tier, is the mt_ prefix needed in the function
> name? I’m fine with keeping it as is, but I just wanted to clarify.

You're right, demote implies some memory tiers. But I like the mt_
prefix, some functions in memory-tiers.c already have this prefix so it
adds consistency: so since you don't mind, I'll keep it :)

>
> Otherwise it LGTM
>
> Reviewed by: Donet Tom <donettom@linux.ibm.com>

Thanks for your time!

Alex

>
>> +                  struct pglist_data *pgdat,
>> +                  struct mem_cgroup *memcg)
>> +{
>> +    int target_nid;
>> +    unsigned int nr_succeeded;
>> +    nodemask_t allowed_mask;
>> +
>> +    struct migration_target_control mtc = {
>> +        /*
>> +         * Allocate from 'node', or fail quickly and quietly.
>> +         * When this happens, 'page' will likely just be discarded
>> +         * instead of migrated.
>> +         */
>> +        .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>> +            __GFP_NOMEMALLOC | GFP_NOWAIT,
>> +        .nmask = &allowed_mask,
>> +        .reason = MR_DEMOTION,
>> +    };
>> +
>> +    if (list_empty(demote_folios))
>> +        return 0;
>> +
>> +    node_get_allowed_targets(pgdat, &allowed_mask);
>> +    mem_cgroup_node_filter_allowed(memcg, &allowed_mask);
>> +    if (nodes_empty(allowed_mask))
>> +        return 0;
>> +
>> +    target_nid = next_demotion_node(pgdat->node_id, &allowed_mask);
>> +    if (target_nid == NUMA_NO_NODE)
>> +        /* No lower-tier nodes or nodes were hot-unplugged. */
>> +        return 0;
>> +
>> +    mtc.nid = target_nid;
>> +
>> +    /* Demotion ignores all cpuset and mempolicy settings */
>> +    migrate_pages(demote_folios, alloc_demote_folio, NULL,
>> +            (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
>> +            &nr_succeeded);
>> +
>> +    return nr_succeeded;
>> +}
>> +
>>   static void disable_all_demotion_targets(void)
>>   {
>>       struct memory_tier *memtier;
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 0fc9373e8251..5e0138b94480 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -983,84 +983,6 @@ static void folio_check_dirty_writeback(struct
>> folio *folio,
>>           mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
>>   }
>>   -static struct folio *alloc_demote_folio(struct folio *src,
>> -        unsigned long private)
>> -{
>> -    struct folio *dst;
>> -    nodemask_t *allowed_mask;
>> -    struct migration_target_control *mtc;
>> -
>> -    mtc = (struct migration_target_control *)private;
>> -
>> -    allowed_mask = mtc->nmask;
>> -    /*
>> -     * make sure we allocate from the target node first also trying to
>> -     * demote or reclaim pages from the target node via kswapd if we
>> are
>> -     * low on free memory on target node. If we don't do this and if
>> -     * we have free memory on the slower(lower) memtier, we would start
>> -     * allocating pages from slower(lower) memory tiers without even
>> forcing
>> -     * a demotion of cold pages from the target memtier. This can
>> result
>> -     * in the kernel placing hot pages in slower(lower) memory tiers.
>> -     */
>> -    mtc->nmask = NULL;
>> -    mtc->gfp_mask |= __GFP_THISNODE;
>> -    dst = alloc_migration_target(src, (unsigned long)mtc);
>> -    if (dst)
>> -        return dst;
>> -
>> -    mtc->gfp_mask &= ~__GFP_THISNODE;
>> -    mtc->nmask = allowed_mask;
>> -
>> -    return alloc_migration_target(src, (unsigned long)mtc);
>> -}
>> -
>> -/*
>> - * Take folios on @demote_folios and attempt to demote them to
>> another node.
>> - * Folios which are not demoted are left on @demote_folios.
>> - */
>> -static unsigned int demote_folio_list(struct list_head *demote_folios,
>> -                      struct pglist_data *pgdat,
>> -                      struct mem_cgroup *memcg)
>> -{
>> -    int target_nid;
>> -    unsigned int nr_succeeded;
>> -    nodemask_t allowed_mask;
>> -
>> -    struct migration_target_control mtc = {
>> -        /*
>> -         * Allocate from 'node', or fail quickly and quietly.
>> -         * When this happens, 'page' will likely just be discarded
>> -         * instead of migrated.
>> -         */
>> -        .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) |
>> -            __GFP_NOMEMALLOC | GFP_NOWAIT,
>> -        .nmask = &allowed_mask,
>> -        .reason = MR_DEMOTION,
>> -    };
>> -
>> -    if (list_empty(demote_folios))
>> -        return 0;
>> -
>> -    node_get_allowed_targets(pgdat, &allowed_mask);
>> -    mem_cgroup_node_filter_allowed(memcg, &allowed_mask);
>> -    if (nodes_empty(allowed_mask))
>> -        return 0;
>> -
>> -    target_nid = next_demotion_node(pgdat->node_id, &allowed_mask);
>> -    if (target_nid == NUMA_NO_NODE)
>> -        /* No lower-tier nodes or nodes were hot-unplugged. */
>> -        return 0;
>> -
>> -    mtc.nid = target_nid;
>> -
>> -    /* Demotion ignores all cpuset and mempolicy settings */
>> -    migrate_pages(demote_folios, alloc_demote_folio, NULL,
>> -              (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
>> -              &nr_succeeded);
>> -
>> -    return nr_succeeded;
>> -}
>> -
>>   static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask)
>>   {
>>       if (gfp_mask & __GFP_FS)
>> @@ -1573,7 +1495,7 @@ static unsigned int shrink_folio_list(struct
>> list_head *folio_list,
>>       /* 'folio_list' is always empty here */
>>         /* Migrate folios selected for demotion */
>> -    nr_demoted = demote_folio_list(&demote_folios, pgdat, memcg);
>> +    nr_demoted = mt_demote_folios(&demote_folios, pgdat, memcg);
>>       nr_reclaimed += nr_demoted;
>>       stat->nr_demoted += nr_demoted;
>>       /* Folios that could not be demoted are still in @demote_folios */