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

[Vlastimil Babka <vbabka@suse.cz>]

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.

> isolate_freepages_range()->split_map_pages() does the post_alloc_hook
> call. As we certainly don't want to zero pages during compaction, we
> could either pass the gfp_mask/"bool clear" down to that functions and
> handle it in there, or handle it in isolate_freepages_range(), after the
> ->split_map_pages() call. Whatever you prefer.

I'd rather not put it in post_alloc_hook() where the bool would then get checked 
from allocator fast path as well.
Maybe split_map_page() then as it contains a for-cycle already.

> Thanks!
>