linux-kernel - Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and promotion daemon

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Message-ID: <1b6fa3d4-2127-4427-bc6d-cddf6459849f@amd.com>
Date: Fri, 7 Mar 2025 08:57:11 +0530
From: Bharata B Rao <bharata@....com>
To: michael.day@....com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: AneeshKumar.KizhakeVeetil@....com, Hasan.Maruf@....com,
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Subject: Re: [RFC PATCH 2/4] mm: kpromoted: Hot page info collection and
 promotion daemon

On 06-Mar-25 10:52 PM, Mike Day wrote:
> 
> 
> On 3/5/25 23:45, Bharata B Rao wrote:
>> +static void kpromoted_migrate(pg_data_t *pgdat)
>> +{
>> +    int nid = pgdat->node_id;
>> +    struct page_hotness_info *phi;
>> +    struct hlist_node *tmp;
>> +    int nr_bkts = HASH_SIZE(page_hotness_hash);
>> +    int bkt;
>> +
>> +    for (bkt = 0; bkt < nr_bkts; bkt++) {
>> +        mutex_lock(&page_hotness_lock[bkt]);
>> +        hlist_for_each_entry_safe(phi, tmp, &page_hotness_hash[bkt], 
>> hnode) {
>> +            if (phi->hot_node != nid)
>> +                continue;
>> +
>> +            if (page_should_be_promoted(phi)) {
>> +                count_vm_event(KPROMOTED_MIG_CANDIDATE);
>> +                if (!kpromote_page(phi)) {
>> +                    count_vm_event(KPROMOTED_MIG_PROMOTED);
>> +                    hlist_del_init(&phi->hnode);
>> +                    kfree(phi);
>> +                }
>> +            } else {
>> +                /*
>> +                 * Not a suitable page or cold page, stop tracking it.
>> +                 * TODO: Identify cold pages and drive demotion?
>> +                 */
>> +                count_vm_event(KPROMOTED_MIG_DROPPED);
>> +                hlist_del_init(&phi->hnode);
>> +                kfree(phi);
>> +            }
>> +        }
>> +        mutex_unlock(&page_hotness_lock[bkt]);
>> +    }
>> +}
>> +
>> +static struct page_hotness_info *__kpromoted_lookup(unsigned long 
>> pfn, int bkt)
>> +{
>> +    struct page_hotness_info *phi;
>> +
>> +    hlist_for_each_entry(phi, &page_hotness_hash[bkt], hnode) {
> 
> Should this be hlist_for_each_entry_safe(), given that 
> kpromoted_migrate() may be
> running concurrently?

I don't think so because the migration path can't walk the list 
concurrently as the lists are protected by mutex.

Regards,
Bharata.