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Subject: Re: [PATCH] Revert "mm/gup: check page posion status for coredump."
To:     Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Aili Yao <yaoaili@kingsoft.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@suse.com>
References: <20210505135407.31590-1-mhocko@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <bedd4817-5b2e-4041-3b55-f8ebfd98505f@redhat.com>
Date:   Wed, 5 May 2021 15:55:42 +0200
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On 05.05.21 15:54, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> While reviewing http://lkml.kernel.org/r/20210429122519.15183-4-david@redhat.com
> I have crossed d3378e86d182 ("mm/gup: check page posion status for
> coredump.") and noticed that this patch is broken in two ways. First it
> doesn't really prevent hwpoison pages from being dumped because hwpoison
> pages can be marked asynchornously at any time after the check.
> Secondly, and more importantly, the patch introduces a ref count leak
> because get_dump_page takes a reference on the page which is not
> releases.
> 
> It also seems that the patch was merged incorrectly because there were
> follow up changes not included as well as discussions on how to address
> the underlying problem http://lkml.kernel.org/r/57ac524c-b49a-99ec-c1e4-ef5027bfb61b@redhat.com
> 
> Therefore revert the original patch.
> 
> Fixes: d3378e86d182 ("mm/gup: check page posion status for coredump.")
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>   mm/gup.c      |  4 ----
>   mm/internal.h | 20 --------------------
>   2 files changed, 24 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 71e546e279fc..a33abe9048ed 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1592,10 +1592,6 @@ struct page *get_dump_page(unsigned long addr)
>   				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
>   	if (locked)
>   		mmap_read_unlock(mm);
> -
> -	if (ret == 1 && is_page_poisoned(page))
> -		return NULL;
> -
>   	return (ret == 1) ? page : NULL;
>   }
>   #endif /* CONFIG_ELF_CORE */
> diff --git a/mm/internal.h b/mm/internal.h
> index ef5f336f59bd..43c4a2f8d4cc 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -96,26 +96,6 @@ static inline void set_page_refcounted(struct page *page)
>   	set_page_count(page, 1);
>   }
>   
> -/*
> - * When kernel touch the user page, the user page may be have been marked
> - * poison but still mapped in user space, if without this page, the kernel
> - * can guarantee the data integrity and operation success, the kernel is
> - * better to check the posion status and avoid touching it, be good not to
> - * panic, coredump for process fatal signal is a sample case matching this
> - * scenario. Or if kernel can't guarantee the data integrity, it's better
> - * not to call this function, let kernel touch the poison page and get to
> - * panic.
> - */
> -static inline bool is_page_poisoned(struct page *page)
> -{
> -	if (PageHWPoison(page))
> -		return true;
> -	else if (PageHuge(page) && PageHWPoison(compound_head(page)))
> -		return true;
> -
> -	return false;
> -}
> -
>   extern unsigned long highest_memmap_pfn;
>   
>   /*
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb