linux-kernel - Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose refcount is one

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Date:   Tue, 12 Jan 2021 12:11:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Muchun Song <songmuchun@...edance.com>, mike.kravetz@...cle.com,
        akpm@...ux-foundation.org
Cc:     n-horiguchi@...jp.nec.com, ak@...ux.intel.com, mhocko@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH v3 1/6] mm: migrate: do not migrate HugeTLB page whose
 refcount is one

On 12.01.21 12:00, David Hildenbrand wrote:
> On 10.01.21 13:40, Muchun Song wrote:
>> If the refcount is one when it is migrated, it means that the page
>> was freed from under us. So we are done and do not need to migrate.
>>
>> This optimization is consistent with the regular pages, just like
>> unmap_and_move() does.
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
>> Acked-by: Yang Shi <shy828301@...il.com>
>> ---
>>  mm/migrate.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 4385f2fb5d18..a6631c4eb6a6 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1279,6 +1279,12 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>>  		return -ENOSYS;
>>  	}
>>  
>> +	if (page_count(hpage) == 1) {
>> +		/* page was freed from under us. So we are done. */
>> +		putback_active_hugepage(hpage);
>> +		return MIGRATEPAGE_SUCCESS;
>> +	}
>> +
>>  	new_hpage = get_new_page(hpage, private);
>>  	if (!new_hpage)
>>  		return -ENOMEM;
>>
> 
> Question: What if called via alloc_contig_range() where we even want to
> "migrate" free pages, meaning, relocate it?
> 

To be more precise:

a) We don't have dissolve_free_huge_pages() calls on the
alloc_contig_range() path. So we *need* migration IIUC.

b) dissolve_free_huge_pages() will fail if going below the reservation.
In that case we really want to migrate free pages. This even applies to
memory offlining.

Either I am missing something important or this patch is more dangerous
than it looks like.

-- 
Thanks,

David / dhildenb