linux-kernel - Re: [PATCH -mm v2] mm/page_isolation: fix potential warning from user

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Date:   Mon, 20 Jan 2020 08:38:12 -0500
From:   Qian Cai <cai@....pw>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v2] mm/page_isolation: fix potential warning from user



> On Jan 20, 2020, at 8:30 AM, David Hildenbrand <david@...hat.com> wrote:
> 
> On 20.01.20 14:19, Qian Cai wrote:
>> It makes sense to call the WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE)
>> from start_isolate_page_range(), but should avoid triggering it from
>> userspace, i.e, from is_mem_section_removable() because it could be a
>> DoS if warn_on_panic is set.
>> 
>> 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>
>> ---
>> 
>> v2: Improve the commit log.
>>    Warn for all start_isolate_page_range() users not just offlining.
>> 
>> mm/page_alloc.c     | 11 ++++-------
>> mm/page_isolation.c | 30 +++++++++++++++++-------------
>> 2 files changed, 21 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..31f5516f5d54 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)
> 
> Why this change?
> 
>> {
>> -	struct page *unmovable = NULL;
>> +	struct page *unmovable = ERR_PTR(-EBUSY);
> 
> Also, why this change?
> 
>> 	struct zone *zone;
>> 	unsigned long flags;
>> -	int ret = -EBUSY;
>> 
>> 	zone = page_zone(page);
>> 
>> @@ -49,21 +49,25 @@ 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 {
>> +		WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
>> +
>> +		if ((isol_flags & REPORT_FAILURE) && !IS_ERR(unmovable))
>> +			/*
> 
> Why this change? (!IS_ERR)
> 
> 
> Some things here look unrelated - or I am missing something :)

The original “ret” variable looks ugly to me, so I just removed that and consolidated with
the “unmovable” pointer to always be able to report an error. Since this cleanup is really
small, I did not bother send a separate patch for it.