Re: [PATCH v9] mm: compaction: handle incorrect MIGRATE_UNMOVABLE type pageblocks

[Minchan Kim <minchan@kernel.org>]

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On 06/06/2012 07:06 PM, Bartlomiej Zolnierkiewicz wrote:

> On Tuesday 05 June 2012 04:38:53 Minchan Kim wrote:
>> On 06/05/2012 10:59 AM, Minchan Kim wrote:
>>
>>> On 06/05/2012 05:22 AM, KOSAKI Motohiro wrote:
>>>
>>>>> +/*
>>>>> + * Returns true if MIGRATE_UNMOVABLE pageblock can be successfully
>>>>> + * converted to MIGRATE_MOVABLE type, false otherwise.
>>>>> + */
>>>>> +static bool can_rescue_unmovable_pageblock(struct page *page, bool
>>>>> locked)
>>>>> +{
>>>>> +    unsigned long pfn, start_pfn, end_pfn;
>>>>> +    struct page *start_page, *end_page, *cursor_page;
>>>>> +
>>>>> +    pfn = page_to_pfn(page);
>>>>> +    start_pfn = pfn&  ~(pageblock_nr_pages - 1);
>>>>> +    end_pfn = start_pfn + pageblock_nr_pages - 1;
>>>>> +
>>>>> +    start_page = pfn_to_page(start_pfn);
>>>>> +    end_page = pfn_to_page(end_pfn);
>>>>> +
>>>>> +    for (cursor_page = start_page, pfn = start_pfn; cursor_page<=
>>>>> end_page;
>>>>> +        pfn++, cursor_page++) {
>>>>> +        struct zone *zone = page_zone(start_page);
>>>>> +        unsigned long flags;
>>>>> +
>>>>> +        if (!pfn_valid_within(pfn))
>>>>> +            continue;
>>>>> +
>>>>> +        /* Do not deal with pageblocks that overlap zones */
>>>>> +        if (page_zone(cursor_page) != zone)
>>>>> +            return false;
>>>>> +
>>>>> +        if (!locked)
>>>>> +            spin_lock_irqsave(&zone->lock, flags);
>>>>> +
>>>>> +        if (PageBuddy(cursor_page)) {
>>>>> +            int order = page_order(cursor_page);
>>>>>
>>>>> -/* Returns true if the page is within a block suitable for migration
>>>>> to */
>>>>> -static bool suitable_migration_target(struct page *page)
>>>>> +            pfn += (1<<  order) - 1;
>>>>> +            cursor_page += (1<<  order) - 1;
>>>>> +
>>>>> +            if (!locked)
>>>>> +                spin_unlock_irqrestore(&zone->lock, flags);
>>>>> +            continue;
>>>>> +        } else if (page_count(cursor_page) == 0 ||
>>>>> +               PageLRU(cursor_page)) {
>>>>> +            if (!locked)
>>>>> +                spin_unlock_irqrestore(&zone->lock, flags);
>>>>> +            continue;
>>>>> +        }
>>>>> +
>>>>> +        if (!locked)
>>>>> +            spin_unlock_irqrestore(&zone->lock, flags);
>>>>> +
>>>>> +        return false;
>>>>> +    }
>>>>> +
>>>>> +    return true;
>>>>> +}
>>>>
>>>> Minchan, are you interest this patch? If yes, can you please rewrite it?
>>>
>>>
>>> Can do it but I want to give credit to Bartlomiej.
>>> Bartlomiej, if you like my patch, could you resend it as formal patch after you do broad testing?
> 
> Sure.


Please use attached one instead of buggy old version. :(
This patch fix THP racing, remove unnecessary lock and add more comment.

> 
>>>> This one are
>>>> not fixed our pointed issue and can_rescue_unmovable_pageblock() still
>>>> has plenty bugs.
>>>> We can't ack it.
>>>>
>>>
>>>
>>> Frankly speaking, I don't want to merge it without any data which prove it's really good for real practice.
>>>
>>> When the patch firstly was submitted, it wasn't complicated so I was okay at that time but it has been complicated
>>> than my expectation. So if Andrew might pass the decision to me, I'm totally NACK if author doesn't provide
>>> any real data or VOC of some client.
> 
> I found this issue by accident while testing compaction code so unfortunately
> I don't have any real bugreport to back it up.  It is just a corner case which
> is more likely to happen in situation where there is rather small number of
> pageblocks and quite heavy kernel memory allocation/freeing activity in
> kernel going on.  I would presume that the issue can happen on some embedded
> configurations but they are not your typical machine and it is not likely
> to see a real bugreport for it.
> 
> I'm also quite unhappy with the increasing complexity of what seemed as
> a quite simple fix initially and I tend to agree that the patch may stay
> out-of-tree until there is a more proven need for it (maybe with documenting
> the issue in the code for the time being).  Still, I would like to have
> all outstanding issues fixed so I can merge the patch locally (and to -mm
> if Andrew agrees) and just wait to see if the patch is ever needed in
> practice.
> 
> I've attached the code that I use to trigger the issue at the bottom of this
> mail so people can reproduce the problem and see for themselves whether it
> is worth to fix it or not.
> 
>>> 1) Any comment?
>>>
>>> Anyway, I fixed some bugs and clean up something I found during review.
> 
> Thanks for doing this.
> 
>>> Minor thing.
>>> 1. change smt_result naming - I never like such long non-consistent naming. How about this?
>>> 2. fix can_rescue_unmovable_pageblock 
>>>    2.1 pfn valid check for page_zone
>>>
>>> Major thing.
>>>
>>>    2.2 add lru_lock for stablizing PageLRU
>>>        If we don't hold lru_lock, there is possibility that unmovable(non-LRU) page can put in movable pageblock.
>>>        It can make compaction/CMA's regression. But there is a concern about deadlock between lru_lock and lock.
>>>        As I look the code, I can't find allocation trial with holding lru_lock so it might be safe(but not sure,
>>>        I didn't test it. It need more careful review/testing) but it makes new locking dependency(not sure, too.
>>>        We already made such rule but I didn't know that until now ;-) ) Why I thought so is we can allocate
>>>        GFP_ATOMIC with holding lru_lock, logically which might be crazy idea.
>>>
>>>    2.3 remove zone->lock in first phase.
>>>        We do rescue unmovable pageblock by 2-phase. In first-phase, we just peek pages so we don't need locking.
>>>        If we see non-stablizing value, it would be caught by 2-phase with needed lock or 
>>>        can_rescue_unmovable_pageblock can return out of loop by stale page_order(cursor_page).
>>>        It couldn't make unmovable pageblock to movable but we can do it next time, again.
>>>        It's not critical.
>>>
>>> 2) Any comment?
>>>
>>> Now I can't inline the code so sorry but attach patch.
>>> It's not a formal patch/never tested.
>>>
>>
>>
>> Attached patch has a BUG in can_rescue_unmovable_pageblock.
>> Resend. I hope it is fixed.
> 
> @@ -399,10 +399,14 @@
>  		} else if (page_count(cursor_page) == 0) {
>  			continue;
>  		} else if (PageLRU(cursor_page)) {
> -			if (!lru_locked && need_lrulock) {
> +			if (!need_lrulock)
> +				continue;
> +			else if (lru_locked)
> +				continue;
> +			else {
>  				spin_lock(&zone->lru_lock);
>  				lru_locked = true;
> -				if (PageLRU(cursor_page))
> +				if (PageLRU(page))
>  					continue;
>  			}
>  		}
> 
> Could you please explain why do we need to check page and not cursor_page
> here?


Slaps self.
That's because I was brain-dead typo.

Please consider attached one and of course, it's totally untested. :(

> 
> Best regards,
> --
> Bartlomiej Zolnierkiewicz
> Samsung Poland R&D Center



> -- 

Kind regards,
Minchan Kim

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