linux-kernel - Re: [PATCH] mm/hugetl.c: keep the page mapping info when free_huge_page() hit the VM_BUG_ON

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Message-ID: <20181113130433.GB16182@dhcp22.suse.cz>
Date:   Tue, 13 Nov 2018 14:04:33 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yongkai Wu <nic.wuyk@...il.com>
Cc:     mike.kravetz@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetl.c: keep the page mapping info when
 free_huge_page() hit the VM_BUG_ON_PAGE

On Tue 13-11-18 20:38:16, Yongkai Wu wrote:
> It is better to keep page mapping info when free_huge_page() hit the
> VM_BUG_ON_PAGE,
> so we can get more infomation from the coredump for further analysis.

The patch seems to be whitespace damaged. Put that aside, have you
actually seen a case where preservning the page state would help to nail
down any bug.

I am not objecting to the patch, it actually makes some sense to me, I
am just curious about a background motivation.
 
> Signed-off-by: Yongkai Wu <nic_w@....com>
> ---
>  mm/hugetlb.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index c007fb5..ba693bb 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1248,10 +1248,11 @@ void free_huge_page(struct page *page)
>   (struct hugepage_subpool *)page_private(page);
>   bool restore_reserve;
> 
> +        VM_BUG_ON_PAGE(page_count(page), page);
> +        VM_BUG_ON_PAGE(page_mapcount(page), page);
> +
>   set_page_private(page, 0);
>   page->mapping = NULL;
> - VM_BUG_ON_PAGE(page_count(page), page);
> - VM_BUG_ON_PAGE(page_mapcount(page), page);
>   restore_reserve = PagePrivate(page);
>   ClearPagePrivate(page);
> 
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs