linux-kernel - Re: [PATCH v2] mm: page_alloc: ignore init_on_free=1 for debug

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Date:   Mon, 29 Mar 2021 11:23:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Sergei Trofimovich <slyfox@...too.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: page_alloc: ignore init_on_free=1 for
 debug_pagealloc=1

On 27.03.21 19:21, Sergei Trofimovich wrote:
> On !ARCH_SUPPORTS_DEBUG_PAGEALLOC (like ia64) debug_pagealloc=1
> implies page_poison=on:
> 
>      if (page_poisoning_enabled() ||
>           (!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
>            debug_pagealloc_enabled()))
>              static_branch_enable(&_page_poisoning_enabled);
> 
> page_poison=on needs to init_on_free=1.
> 
> Before the change id happened too late for the following case:
> - have PAGE_POISONING=y
> - have page_poison unset
> - have !ARCH_SUPPORTS_DEBUG_PAGEALLOC arch (like ia64)
> - have init_on_free=1
> - have debug_pagealloc=1
> 
> That way we get both keys enabled:
> - static_branch_enable(&init_on_free);
> - static_branch_enable(&_page_poisoning_enabled);
> 
> which leads to poisoned pages returned for __GFP_ZERO pages.
> 
> After the change we execute only:
> - static_branch_enable(&_page_poisoning_enabled);
> and ignore init_on_free=1.
> 
> CC: Vlastimil Babka <vbabka@...e.cz>
> CC: Andrew Morton <akpm@...ux-foundation.org>
> CC: linux-mm@...ck.org
> CC: David Hildenbrand <david@...hat.com>
> CC: Andrey Konovalov <andreyknvl@...il.com>
> Link: https://lkml.org/lkml/2021/3/26/443

Again, Fixes: tag? IOW, which commit initially broke it.

> Signed-off-by: Sergei Trofimovich <slyfox@...too.org>
> ---
>   mm/page_alloc.c | 30 +++++++++++++++++-------------
>   1 file changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d57d9b4f7089..10a8a1d28c11 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -764,32 +764,36 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
>    */
>   void init_mem_debugging_and_hardening(void)
>   {
> +	bool page_poison_requested = page_poisoning_enabled();

s/page_poison_requested/page_poisoning_requested/

And I wonder if you should just initialize to "false" here.

Without CONFIG_PAGE_POISONING, page_poisoning_enabled() will always 
return false, so it seems unnecessary.

> +
> +#ifdef CONFIG_PAGE_POISONING
> +	/*
> +	 * Page poisoning is debug page alloc for some arches. If
> +	 * either of those options are enabled, enable poisoning.
> +	 */
> +	if (page_poisoning_enabled() ||
> +	     (!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
> +	      debug_pagealloc_enabled())) {
> +		static_branch_enable(&_page_poisoning_enabled);
> +		page_poison_requested = true;
> +	}
> +#endif

Apart from that, looks good.


-- 
Thanks,

David / dhildenb