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Subject: Re: [PATCH v1] mm/page_alloc: clear pages in alloc_contig_pages()
 with init_on_alloc=1 or __GFP_ZERO
To:     Vlastimil Babka <vbabka@suse.cz>, Michal Hocko <mhocko@suse.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Oscar Salvador <osalvador@suse.de>,
        Kees Cook <keescook@chromium.org>,
        Michael Ellerman <mpe@ellerman.id.au>
References: <20201110193240.25401-1-david@redhat.com>
 <20201111084738.GT12240@dhcp22.suse.cz>
 <4ebc711e-7fbc-62aa-b88f-3d6ffa9379ff@redhat.com>
 <b2d29dc2-cfe9-415d-7037-402dcc0c0f17@suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <5e104380-c0b1-4911-b484-b6e1e1c46f7d@redhat.com>
Date:   Wed, 11 Nov 2020 11:05:21 +0100
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On 11.11.20 10:58, Vlastimil Babka wrote:
> On 11/11/20 10:06 AM, David Hildenbrand wrote:
>> On 11.11.20 09:47, Michal Hocko wrote:
>>> On Tue 10-11-20 20:32:40, David Hildenbrand wrote:
>>>> commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and
>>>> init_on_free=1 boot options") resulted with init_on_alloc=1 in all pages
>>>> leaving the buddy via alloc_pages() and friends to be
>>>> initialized/cleared/zeroed on allocation.
>>>>
>>>> However, the same logic is currently not applied to
>>>> alloc_contig_pages(): allocated pages leaving the buddy aren't cleared
>>>> with init_on_alloc=1 and init_on_free=0. Let's also properly clear
>>>> pages on that allocation path and add support for __GFP_ZERO.
>>>
>>> AFAIR we do not have any user for __GFP_ZERO right? Not that this is
>>
>> Sorry, I had extended information under "---" but accidentally
>> regenerated the patch before sending it out.
>>
>> __GFP_ZERO is not used yet. It's intended to be used in
>> https://lkml.kernel.org/r/20201029162718.29910-1-david@redhat.com
>> and I can move that change into a separate patch if desired.
>>
>>> harmful but it is better to call that explicitly because a missing
>>> implementation would be a real problem and as such a bug fix.
>>>
>>> I am also not sure handling init_on_free at the higher level is good.
>>> As we have discussed recently the primary point of this feature is to
>>> add clearing at very few well defined entry points rather than spill it over
>>> many places. In this case the entry point for the allocator is
>>> __isolate_free_page which removes pages from the page allocator. I
>>> haven't checked how much this is used elsewhere but I would expect
>>> init_on_alloc to be handled there.
>>
>> Well, this is the entry point to our range allocator, which lives in
>> page_alloc.c - used by actual high-level allocators (CMA, gigantic
>> pages, etc). It's just a matter of taste where we want to have that
>> handling exactly inside our allocator.
> 
> I agree alloc_contig_range() is fine as an entry point.

Thanks, let's see if Michal insists of having this somewhere inside 
isolate_freepages_range() instead.

-- 
Thanks,

David / dhildenb