Re: [PATCH] mm: check the return value of lookup_page_ext for all call sites

[Minchan Kim <minchan@kernel.org>]

On Mon, May 23, 2016 at 10:16:08AM -0700, Yang Shi wrote:
> Per the discussion with Joonsoo Kim [1], we need check the return value of
> lookup_page_ext() for all call sites since it might return NULL in some cases,
> although it is unlikely, i.e. memory hotplug.
> 
> Tested with ltp with "page_owner=0".
> 
> [1] http://lkml.kernel.org/r/20160519002809.GA10245@js1304-P5Q-DELUXE
> 
> Signed-off-by: Yang Shi <yang.shi@linaro.org>

I didn't read code code in detail to see how page_ext memory space
allocated in boot code and memory hotplug but to me, it's not good
to check NULL whenever we calls lookup_page_ext.

More dangerous thing is now page_ext is used by optionable feature(ie, not
critical for system stability) but if we want to use page_ext as
another important tool for the system in future,
it could be a serious problem.

Can we put some hooks of page_ext into memory-hotplug so guarantee
that page_ext memory space is allocated with memmap space at the
same time? IOW, once every PFN wakers find a page is valid, page_ext
is valid, too so lookup_page_ext never returns NULL on valid page
by design.

I hope we consider this direction, too.

Thanks.

> ---
>  include/linux/page_idle.h | 43 ++++++++++++++++++++++++++++++++++++-------
>  mm/page_alloc.c           |  6 ++++++
>  mm/page_owner.c           | 27 +++++++++++++++++++++++++++
>  mm/page_poison.c          |  8 +++++++-
>  mm/vmstat.c               |  2 ++
>  5 files changed, 78 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> index bf268fa..8f5d4ad 100644
> --- a/include/linux/page_idle.h
> +++ b/include/linux/page_idle.h
> @@ -46,33 +46,62 @@ extern struct page_ext_operations page_idle_ops;
>  
>  static inline bool page_is_young(struct page *page)
>  {
> -	return test_bit(PAGE_EXT_YOUNG, &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return false;
> +
> +	return test_bit(PAGE_EXT_YOUNG, &page_ext->flags);
>  }
>  
>  static inline void set_page_young(struct page *page)
>  {
> -	set_bit(PAGE_EXT_YOUNG, &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return;
> +
> +	set_bit(PAGE_EXT_YOUNG, &page_ext->flags);
>  }
>  
>  static inline bool test_and_clear_page_young(struct page *page)
>  {
> -	return test_and_clear_bit(PAGE_EXT_YOUNG,
> -				  &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return false;
> +
> +	return test_and_clear_bit(PAGE_EXT_YOUNG, &page_ext->flags);
>  }
>  
>  static inline bool page_is_idle(struct page *page)
>  {
> -	return test_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return false;
> +
> +	return test_bit(PAGE_EXT_IDLE, &page_ext->flags);
>  }
>  
>  static inline void set_page_idle(struct page *page)
>  {
> -	set_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return;
> +
> +	set_bit(PAGE_EXT_IDLE, &page_ext->flags);
>  }
>  
>  static inline void clear_page_idle(struct page *page)
>  {
> -	clear_bit(PAGE_EXT_IDLE, &lookup_page_ext(page)->flags);
> +	struct page_ext *page_ext;
> +	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)
> +		return;
> +
> +	clear_bit(PAGE_EXT_IDLE, &page_ext->flags);
>  }
>  #endif /* CONFIG_64BIT */
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f8f3bfc..d27e8b9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -656,6 +656,9 @@ static inline void set_page_guard(struct zone *zone, struct page *page,
>  		return;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		return;
> +
>  	__set_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);
>  
>  	INIT_LIST_HEAD(&page->lru);
> @@ -673,6 +676,9 @@ static inline void clear_page_guard(struct zone *zone, struct page *page,
>  		return;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		return;
> +
>  	__clear_bit(PAGE_EXT_DEBUG_GUARD, &page_ext->flags);
>  
>  	set_page_private(page, 0);
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 792b56d..902e398 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -55,6 +55,8 @@ void __reset_page_owner(struct page *page, unsigned int order)
>  
>  	for (i = 0; i < (1 << order); i++) {
>  		page_ext = lookup_page_ext(page + i);
> +		if (unlikely(!page_ext))
> +			continue;
>  		__clear_bit(PAGE_EXT_OWNER, &page_ext->flags);
>  	}
>  }
> @@ -62,6 +64,10 @@ void __reset_page_owner(struct page *page, unsigned int order)
>  void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
>  {
>  	struct page_ext *page_ext = lookup_page_ext(page);
> +
> +	if (unlikely(!page_ext))
> +		return;
> +
>  	struct stack_trace trace = {
>  		.nr_entries = 0,
>  		.max_entries = ARRAY_SIZE(page_ext->trace_entries),
> @@ -82,6 +88,8 @@ void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
>  void __set_page_owner_migrate_reason(struct page *page, int reason)
>  {
>  	struct page_ext *page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		return;
>  
>  	page_ext->last_migrate_reason = reason;
>  }
> @@ -89,6 +97,12 @@ void __set_page_owner_migrate_reason(struct page *page, int reason)
>  gfp_t __get_page_owner_gfp(struct page *page)
>  {
>  	struct page_ext *page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		/*
> +		 * The caller just returns 0 if no valid gfp
> +		 * So return 0 here too.
> +		 */
> +		return 0;
>  
>  	return page_ext->gfp_mask;
>  }
> @@ -97,6 +111,10 @@ void __copy_page_owner(struct page *oldpage, struct page *newpage)
>  {
>  	struct page_ext *old_ext = lookup_page_ext(oldpage);
>  	struct page_ext *new_ext = lookup_page_ext(newpage);
> +
> +	if (unlikely(!old_ext || !new_ext))
> +		return;
> +
>  	int i;
>  
>  	new_ext->order = old_ext->order;
> @@ -186,6 +204,11 @@ err:
>  void __dump_page_owner(struct page *page)
>  {
>  	struct page_ext *page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext)) {
> +		pr_alert("There is not page extension available.\n");
> +		return;
> +	}
> +
>  	struct stack_trace trace = {
>  		.nr_entries = page_ext->nr_entries,
>  		.entries = &page_ext->trace_entries[0],
> @@ -251,6 +274,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
>  		}
>  
>  		page_ext = lookup_page_ext(page);
> +		if (unlikely(!page_ext))
> +			continue;
>  
>  		/*
>  		 * Some pages could be missed by concurrent allocation or free,
> @@ -317,6 +342,8 @@ static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
>  				continue;
>  
>  			page_ext = lookup_page_ext(page);
> +			if (unlikely(!page_ext))
> +				continue;
>  
>  			/* Maybe overraping zone */
>  			if (test_bit(PAGE_EXT_OWNER, &page_ext->flags))
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index 1eae5fa..2e647c6 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -54,6 +54,9 @@ static inline void set_page_poison(struct page *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		return;
> +
>  	__set_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
>  }
>  
> @@ -62,6 +65,9 @@ static inline void clear_page_poison(struct page *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> +	if (unlikely(!page_ext))
> +		return;
> +
>  	__clear_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
>  }
>  
> @@ -70,7 +76,7 @@ bool page_is_poisoned(struct page *page)
>  	struct page_ext *page_ext;
>  
>  	page_ext = lookup_page_ext(page);
> -	if (!page_ext)
> +	if (unlikely(!page_ext))
>  		return false;
>  
>  	return test_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 77e42ef..cb2a67b 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1061,6 +1061,8 @@ static void pagetypeinfo_showmixedcount_print(struct seq_file *m,
>  				continue;
>  
>  			page_ext = lookup_page_ext(page);
> +			if (unlikely(!page_ext))
> +				continue;
>  
>  			if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
>  				continue;
> -- 
> 2.0.2
> 
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