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

[David Hildenbrand]

On 20.01.20 14:56, Qian Cai wrote:
> 
> 
>> On Jan 20, 2020, at 8:38 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>> On 20.01.20 14:30, David Hildenbrand 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@xxxxxxxxxx>
>>>> Signed-off-by: Qian Cai <cai@xxxxxx>
>>>> ---
>>>>
>>>> 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 :)
>>>
>>
>> FWIW, I'd prefer this change without any such cleanups (e.g., I don't
>> like returning a bool from this function and the IS_ERR handling, makes
>> the function harder to read than before)
> 
> What is Michal or Andrewâs opinion? BTW, a bonus point to return a bool
> is that it helps the code robustness in general, as UBSAN will be able to
> catch any abuse.
> 

A return type of bool on a function that does not test a property
("has_...", "is"...") is IMHO confusing.

If we have an int, it is clear that "0" means "success". With a bool
(true/false), it is not clear.

-- 
Thanks,

David / dhildenb