Re: [PATCH v1] mm/page_alloc: clear pages in alloc_contig_pages() with init_on_alloc=1 or __GFP_ZERO

[David Hildenbrand <david@redhat.com>]

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