linux-kernel - Re: [Patch v4 5/7] mm/hugetlb: a page from buddy is not on any list

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Date:   Wed, 2 Sep 2020 10:25:47 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, bhe@...hat.com
Subject: Re: [Patch v4 5/7] mm/hugetlb: a page from buddy is not on any list

On 9/2/20 3:49 AM, Vlastimil Babka wrote:
> On 9/1/20 3:46 AM, Wei Yang wrote:
>> The page allocated from buddy is not on any list, so just use list_add()
>> is enough.
>>
>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>> Reviewed-by: Baoquan He <bhe@...hat.com>
>> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
>> ---
>>  mm/hugetlb.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 441b7f7c623e..c9b292e664c4 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -2405,7 +2405,7 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
>>  			h->resv_huge_pages--;
>>  		}
>>  		spin_lock(&hugetlb_lock);
>> -		list_move(&page->lru, &h->hugepage_activelist);
>> +		list_add(&page->lru, &h->hugepage_activelist);
> 
> Hmm, how does that list_move() actually not crash today?
> Page has been taken from free lists, thus there was list_del() and page->lru
> should be poisoned.
> list_move() does __list_del_entry() which will either detect the poison with
> CONFIG_DEBUG_LIST, or crash accessing the poison, no?
> Am I missing something or does it mean this code is actually never executed in wild?
> 

There is not enough context in the diff, but the hugetlb page was not taken
from the free list.  Rather, it was just created by a call to
alloc_buddy_huge_page_with_mpol().  As part of the allocation/creation
prep_new_huge_page will be called which will INIT_LIST_HEAD(&page->lru).

-- 
Mike Kravetz