linux-kernel - Re: [PATCH -mm] mm/page_isolation: fix potential warning from user

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Date:   Mon, 20 Jan 2020 09:50:33 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org
Cc:     mhocko@...nel.org, david@...hat.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm] mm/page_isolation: fix potential warning from user

Hello Qian,

On 01/20/2020 09:12 AM, Qian Cai wrote:
> It makes sense to call the WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE)
> from the offlining path, but should avoid triggering it from userspace,
> i.e, from is_mem_section_removable().

Could you elaborate why it makes sense not to warn about an unmovable
ZONE_MOVABLE page when an user tries to query about a memory block
device's movability through sysfs ?

> 
> While at it, simplify the code a bit by removing an unnecessary jump
> label and a local variable, so set_migratetype_isolate() could really
> return a bool.
> 
> Suggested-by: Michal Hocko <mhocko@...nel.org>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  mm/page_alloc.c     | 11 ++++-------
>  mm/page_isolation.c | 31 ++++++++++++++++++-------------
>  2 files changed, 22 insertions(+), 20 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 621716a25639..3c4eb750a199 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8231,7 +8231,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  		if (is_migrate_cma(migratetype))
>  			return NULL;
>  
> -		goto unmovable;
> +		return page;
>  	}
>  
>  	for (; iter < pageblock_nr_pages; iter++) {
> @@ -8241,7 +8241,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  		page = pfn_to_page(pfn + iter);
>  
>  		if (PageReserved(page))
> -			goto unmovable;
> +			return page;
>  
>  		/*
>  		 * If the zone is movable and we have ruled out all reserved
> @@ -8261,7 +8261,7 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  			unsigned int skip_pages;
>  
>  			if (!hugepage_migration_supported(page_hstate(head)))
> -				goto unmovable;
> +				return page;
>  
>  			skip_pages = compound_nr(head) - (page - head);
>  			iter += skip_pages - 1;
> @@ -8303,12 +8303,9 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  		 * is set to both of a memory hole page and a _used_ kernel
>  		 * page at boot.
>  		 */
> -		goto unmovable;
> +		return page;
>  	}
>  	return NULL;
> -unmovable:
> -	WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
> -	return pfn_to_page(pfn + iter);
>  }
>  
>  #ifdef CONFIG_CONTIG_ALLOC
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index e70586523ca3..97f673d5fefa 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -15,12 +15,12 @@
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/page_isolation.h>
>  
> -static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags)
> +static bool set_migratetype_isolate(struct page *page, int migratetype,
> +				    int isol_flags)
>  {
> -	struct page *unmovable = NULL;
> +	struct page *unmovable = ERR_PTR(-EBUSY);
>  	struct zone *zone;
>  	unsigned long flags;
> -	int ret = -EBUSY;
>  
>  	zone = page_zone(page);
>  
> @@ -49,21 +49,26 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>  									NULL);
>  
>  		__mod_zone_freepage_state(zone, -nr_pages, mt);
> -		ret = 0;
>  	}
>  
>  out:
>  	spin_unlock_irqrestore(&zone->lock, flags);
> -	if (!ret)
> +
> +	if (!unmovable) {
>  		drain_all_pages(zone);
> -	else if ((isol_flags & REPORT_FAILURE) && unmovable)
> -		/*
> -		 * printk() with zone->lock held will guarantee to trigger a
> -		 * lockdep splat, so defer it here.
> -		 */
> -		dump_page(unmovable, "unmovable page");
> -
> -	return ret;
> +	} else {
> +		if (isol_flags & MEMORY_OFFLINE)
> +			WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);> +
> +		if ((isol_flags & REPORT_FAILURE) && !IS_ERR(unmovable))
> +			/*
> +			 * printk() with zone->lock held will likely trigger a
> +			 * lockdep splat, so defer it here.
> +			 */
> +			dump_page(unmovable, "unmovable page");
> +	}
> +
> +	return !!unmovable;
>  }
>  
>  static void unset_migratetype_isolate(struct page *page, unsigned migratetype)

set_migratetype_isolate() gets called from CMA as well as HugeTLB
allocation paths, so its not only during offline. Hence the commit
message should be changed to reflect this.

- Anshuman

>