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

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Message-ID: <d38527b5-140d-15e5-c1c4-f381602eab46@redhat.com>
Date:   Wed, 10 Feb 2021 15:11:05 +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 15:09, 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 :) )

-- 
Thanks,

David / dhildenb