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Date:   Mon, 17 Jun 2019 09:46:54 +0300
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     "Alastair D'Silva" <alastair@au1.ibm.com>
Cc:     alastair@d-silva.org, Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.com>,
        Michal Hocko <mhocko@suse.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Wei Yang <richard.weiyang@gmail.com>, Qian Cai <cai@lca.pw>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Jiri Kosina <jkosina@suse.cz>,
        Mukesh Ojha <mojha@codeaurora.org>,
        Arun KS <arunks@codeaurora.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Baoquan He <bhe@redhat.com>,
        Logan Gunthorpe <logang@deltatee.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/5] mm: Trigger bug on if a section is not found in
 __section_nr
References: <20190617043635.13201-1-alastair@au1.ibm.com>
 <20190617043635.13201-2-alastair@au1.ibm.com>
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On Mon, Jun 17, 2019 at 02:36:27PM +1000, Alastair D'Silva wrote:
> From: Alastair D'Silva <alastair@d-silva.org>
> 
> If a memory section comes in where the physical address is greater than
> that which is managed by the kernel, this function would not trigger the
> bug and instead return a bogus section number.
> 
> This patch tracks whether the section was actually found, and triggers the
> bug if not.
> 
> Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> ---
>  mm/sparse.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/sparse.c b/mm/sparse.c
> index fd13166949b5..104a79fedd00 100644
> --- a/mm/sparse.c
> +++ b/mm/sparse.c
> @@ -105,20 +105,23 @@ static inline int sparse_index_init(unsigned long section_nr, int nid)
>  int __section_nr(struct mem_section* ms)
>  {
>  	unsigned long root_nr;
> -	struct mem_section *root = NULL;
> +	struct mem_section *found = NULL;
> +	struct mem_section *root;
> 
>  	for (root_nr = 0; root_nr < NR_SECTION_ROOTS; root_nr++) {
>  		root = __nr_to_section(root_nr * SECTIONS_PER_ROOT);
>  		if (!root)
>  			continue;
> 
> -		if ((ms >= root) && (ms < (root + SECTIONS_PER_ROOT)))
> -		     break;
> +		if ((ms >= root) && (ms < (root + SECTIONS_PER_ROOT))) {
> +			found = root;
> +			break;
> +		}
>  	}
> 
> -	VM_BUG_ON(!root);
> +	VM_BUG_ON(!found);

Isn't it enough to check for root_nr == NR_SECTION_ROOTS?

> 
> -	return (root_nr * SECTIONS_PER_ROOT) + (ms - root);
> +	return (root_nr * SECTIONS_PER_ROOT) + (ms - found);

It'll still return a bogus section number with CONFIG_DEBUG_VM=n

>  }
>  #else
>  int __section_nr(struct mem_section* ms)
> -- 
> 2.21.0
> 

-- 
Sincerely yours,
Mike.