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

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Message-ID: <fd89ad4a-ef8d-e54e-8769-d6b15c40cc43@suse.cz>
Date:   Wed, 2 Sep 2020 12:49:46 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        mike.kravetz@...cle.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 9/1/20 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?

>  		/* Fall through */

Maybe delete this comment? This is not a switch statement.

>  	}
>  	hugetlb_cgroup_commit_charge(idx, pages_per_huge_page(h), h_cg, page);
>