linux-kernel - Re: [PATCH] mm: check the return value of lookup_page

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Date:	Mon, 30 May 2016 15:11:17 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	"Shi, Yang" <yang.shi@...aro.org>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>, <akpm@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<linaro-kernel@...ts.linaro.org>,
	Tang Chen <tangchen@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] mm: check the return value of lookup_page_ext for all
 call sites

On Fri, May 27, 2016 at 11:16:41AM -0700, Shi, Yang wrote:

<snip>

> >
> >If we goes this way, how to guarantee this race?
> 
> Thanks for pointing out this. It sounds reasonable. However, this
> should be only possible to happen on 32 bit since just 32 bit
> version page_is_idle() calls lookup_page_ext(), it doesn't do it on
> 64 bit.
> 
> And, such race condition should exist regardless of whether DEBUG_VM
> is enabled or not, right?
> 
> rcu might be good enough to protect it.
> 
> A quick fix may look like:
> 
> diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> index 8f5d4ad..bf0cd6a 100644
> --- a/include/linux/page_idle.h
> +++ b/include/linux/page_idle.h
> @@ -77,8 +77,12 @@ static inline bool
> test_and_clear_page_young(struct page *page)
>  static inline bool page_is_idle(struct page *page)
>  {
>         struct page_ext *page_ext;
> +
> +       rcu_read_lock();
>         page_ext = lookup_page_ext(page);
> +       rcu_read_unlock();
> +
> 	if (unlikely(!page_ext))
>                 return false;
> 
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 56b160f..94927c9 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -183,7 +183,6 @@ struct page_ext *lookup_page_ext(struct page *page)
>  {
>         unsigned long pfn = page_to_pfn(page);
>         struct mem_section *section = __pfn_to_section(pfn);
> -#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
>         /*
>          * The sanity checks the page allocator does upon freeing a
>          * page can reach here before the page_ext arrays are
> @@ -195,7 +194,7 @@ struct page_ext *lookup_page_ext(struct page *page)
>          */
>         if (!section->page_ext)
>                 return NULL;
> -#endif
> +
>         return section->page_ext + pfn;
>  }
> 
> @@ -279,7 +278,8 @@ static void __free_page_ext(unsigned long pfn)
>                 return;
>         base = ms->page_ext + pfn;
>         free_page_ext(base);
> -       ms->page_ext = NULL;
> +       rcu_assign_pointer(ms->page_ext, NULL);
> +       synchronize_rcu();

How does it fix the problem?
I cannot understand your point.

>  }
> 
>  static int __meminit online_page_ext(unsigned long start_pfn,
> 
> Thanks,
> Yang
> 
> >
> >                                kpageflags_read
> >                                stable_page_flags
> >                                page_is_idle
> >                                  lookup_page_ext
> >                                  section = __pfn_to_section(pfn)
> >offline_pages
> >memory_notify(MEM_OFFLINE)
> >  offline_page_ext
> >  ms->page_ext = NULL
> >                                  section->page_ext + pfn
> >
> >>
> >>Thanks.
> >>
>