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Subject: Re: [PATCH 2/2] mm: don't hide potentially null memmap pointer in
 sparse_remove_section
To:     Alastair D'Silva <alastair@d-silva.org>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Oscar Salvador <osalvador@suse.de>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Wei Yang <richardw.yang@linux.intel.com>,
        Qian Cai <cai@lca.pw>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20190827053656.32191-1-alastair@au1.ibm.com>
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <4a08e3ec-1859-e4a7-6c08-5f36b09712d4@redhat.com>
Date:   Tue, 27 Aug 2019 09:08:49 +0200
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On 27.08.19 09:00, Alastair D'Silva wrote:
> On Tue, 2019-08-27 at 08:24 +0200, Michal Hocko wrote:
>> On Tue 27-08-19 15:36:55, Alastair D'Silva wrote:
>>> From: Alastair D'Silva <alastair@d-silva.org>
>>>
>>> By adding offset to memmap before passing it in to
>>> clear_hwpoisoned_pages,
>>> we hide a theoretically null memmap from the null check inside
>>> clear_hwpoisoned_pages.
>>
>> Isn't that other way around? Calculating the offset struct page
>> pointer
>> will actually make the null check effective. Besides that I cannot
>> really see how pfn_to_page would return NULL. I have to confess that
>> I
>> cannot really see how offset could lead to a NULL struct page either
>> and
>> I strongly suspect that the NULL check is not really needed. Maybe it
>> used to be in the past.
>>
> 
> You're probably right, but I didn't feel confident in removing the NULL
> check. 
> 
> While the NULL check remains though, I can't see how adding the offset
> would turn a non-NULL pointer into a NULL unless the pointer is invalid
> in the first place, and if this is the case, we should have a comment
> explaining this.
> 
> The NULL check was added in commit:
> 95a4774d055c ("memory-hotplug: update mce_bad_pages when removing the
> memory")
> where memmap was originally inited to NULL, and only conditionally
> given a value.
> 
> With this in mind, since that situation is no longer true, I think we
> could instead drop the NULL check.
> 

Makes sense to me.

-- 

Thanks,

David / dhildenb