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 19:56:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        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 7:25 PM, Mike Kravetz wrote:
> 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).

Ah so indeed I was missing something :) Thanks. Then this is indeed a an
optimization and not a bugfix and doesn't need stable@. Sorry for the noise.