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

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Date:   Wed, 10 Feb 2021 15:22:07 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        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 10.02.21 15:14, Oscar Salvador wrote:
> On Wed, Feb 10, 2021 at 03:11:05PM +0100, David Hildenbrand wrote:
>> On 10.02.21 15:09, Oscar Salvador wrote:
>>> On Wed, Feb 10, 2021 at 09:56:37AM +0100, David Hildenbrand wrote:
>>>> On 08.02.21 11:38, 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)) {
>>>>
>>>> I wonder if we should care about races here. What if someone concurrently
>>>> allocates/frees?
>>>>
>>>> Note that PageHuge() succeeds on tail pages, isolate_huge_page() not, i
>>>> assume we'll have to handle that as well.
>>>>
>>>> I wonder if it would make sense to move some of the magic to hugetlb code
>>>> and handle it there with less chances for races (isolate if used,
>>>> alloc-and-dissolve if not).
>>>
>>> Yes, it makes sense to keep the magic in hugetlb code.
>>> Note, though, that removing all races might be tricky.
>>>
>>> isolate_huge_page() checks for PageHuge under hugetlb_lock,
>>> so there is a race between a call to PageHuge(x) and a subsequent
>>> call to isolate_huge_page().
>>> But we should be fine as isolate_huge_page will fail in case the page is
>>> no longer HugeTLB.
>>>
>>> Also, since isolate_migratepages_block() gets called with ranges
>>> pageblock aligned, we should never be handling tail pages in the core
>>> of the function. E.g: the same way we handle THP:
>>
>> Gigantic pages? (spoiler: see my comments to next patch :) )
> 
> Oh, yeah, that sucks.
> We had the same problem in scan_movable_pages/has_unmovable_pages
> with such pages.
> 
> Uhm, I will try to be more careful :-)
> 

Gigantic pages are a minefield. Not your fault :)

-- 
Thanks,

David / dhildenb