linux-kernel - Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig

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Date:   Thu, 11 Feb 2021 11:40:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig_range handle
 in-use hugetlb pages

On 11.02.21 01:56, Mike Kravetz wrote:
> On 2/8/21 2:38 AM, Oscar Salvador wrote:
>> alloc_contig_range is not prepared to handle hugetlb pages and will
>> fail if it ever sees one, but since they can be migrated as any other
>> page (LRU and Movable), it makes sense to also handle them.
>>
>> For now, do it only when coming from alloc_contig_range.
>>
>> Signed-off-by: Oscar Salvador <osalvador@...e.de>
>> ---
>>   mm/compaction.c | 17 +++++++++++++++++
>>   mm/vmscan.c     |  5 +++--
>>   2 files changed, 20 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index e5acb9714436..89cd2e60da29 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -940,6 +940,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>>   			goto isolate_fail;
>>   		}
>>   
>> +		/*
>> +		 * Handle hugetlb pages only when coming from alloc_contig
>> +		 */
>> +		if (PageHuge(page) && cc->alloc_contig) {
>> +			if (page_count(page)) {
> 
> Thanks for doing this!
> 
> I agree with everything in the discussion you and David had.  This code
> is racy, but since we are scanning lockless there is no way to eliminate
> them all.  Best to just minimize the windows and document.
>

Agreed - and make sure that we don't have strange side. (e.g., in the 
next patch, allocate a new page, try to dissolve. Dissolving fails, what 
happens to the just-allocated page?)

-- 
Thanks,

David / dhildenb