Re: [PATCH 2/2] mm/page_alloc.c: add config option to sanitize freed pages

From: Anisse Astier
Date: Mon Apr 27 2015 - 04:12:20 EST

Hi Andi,

Thinks for taking the time to review this.

On Sun, Apr 26, 2015 at 10:12 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> Anisse Astier <anisse@xxxxxxxxx> writes:
>> + If unsure, say N.
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 05fcec9..c71440a 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -803,6 +803,11 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>> debug_check_no_obj_freed(page_address(page),
>> PAGE_SIZE << order);
>> }
>> +
>> + zero_pages(page, order);
>> +#endif
> And not removing the clear on __GFP_ZERO by remembering that?
> That means all clears would be done twice.
> That patch is far too simple. Clearing is commonly the most
> expensive kernel operation.

I thought about this, but if you unconditionally remove the clear on
__GFP_ZERO, you wouldn't be guaranteed to have a zeroed page when
memory is first used (you would protect the kernel from its own info
leaks though); you'd need to clear memory on boot for example.

If you try to remember that a page it's cleared, it means using a page
flag, which is was previously deemed too precious for this kind of

Regarding the expensive operation, I don't think this is an option
you'd enable on your systems if you care about performance.


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