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

[David Hildenbrand]

On 10.11.20 20:32, 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.
>
> With this change, we will see double clearing of pages in some
> cases. One example are gigantic pages (either allocated via CMA, or
> allocated dynamically via alloc_contig_pages()) - which is the right
> thing to do (and to be optimized outside of the buddy in the callers) as
> discussed in:
>    https://lkml.kernel.org/r/20201019182853.7467-1-gpiccoli@xxxxxxxxxxxxx
>
> This change implies that with init_on_alloc=1
> - All CMA allocations will be cleared
> - Gigantic pages allocated via alloc_contig_pages() will be cleared
> - virtio-mem memory to be unplugged will be cleared. While this is
>    suboptimal, it's similar to memory balloon drivers handling, where
>    all pages to be inflated will get cleared as well.
>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   mm/page_alloc.c | 24 +++++++++++++++++++++---
>   1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index eed4f4075b3c..0361b119b74e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8453,6 +8453,19 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>   	return 0;
>   }
>   
> +static void __alloc_contig_clear_range(unsigned long start_pfn,
> +				       unsigned long end_pfn)
> +{
> +	unsigned long pfn;
> +
> +	for (pfn = start_pfn; pfn < end_pfn; pfn += MAX_ORDER_NR_PAGES) {
> +		cond_resched();
> +		kernel_init_free_pages(pfn_to_page(pfn),
> +				       min_t(unsigned long, end_pfn - pfn,
> +					     MAX_ORDER_NR_PAGES));

In weird cases, we might cross a MAX_ORDER - 1 block here. I'll fix that.

--
Thanks,

David / dhildenb