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

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Date:   Wed, 11 Nov 2020 10:25:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Oscar Salvador <osalvador@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v1] mm/page_alloc: clear pages in alloc_contig_pages()
 with init_on_alloc=1 or __GFP_ZERO

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@canonical.com
> 
> 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@...ux-foundation.org>
> Cc: Alexander Potapenko <glider@...gle.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mike Rapoport <rppt@...ux.ibm.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>   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