Subject: Re: [PATCH v1 08/12] powerpc/pseries: CMM: Implement balloon compaction

From: Michael Ellerman <[email-masked]>
Date: Wed, 13 Nov 2019 22:08:44 +1100

David Hildenbrand <[email-masked]> writes:
> On 12.11.19 11:46, Michael Ellerman wrote:
>> David Hildenbrand <[email-masked]> writes:
>>> On 31.10.19 15:29, David Hildenbrand wrote:
>>>> We can now get rid of the cmm_lock and completely rely on the balloon
>>>> compaction internals, which now also manage the page list and the lock.
>> ...
>>>> +
>>>> +static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
>>>> +			   struct page *newpage, struct page *page,
>>>> +			   enum migrate_mode mode)
>>>> +{
>>>> +	unsigned long flags;
>>>> +
>>>> +	/*
>>>> +	 * loan/"inflate" the newpage first.
>>>> +	 *
>>>> +	 * We might race against the cmm_thread who might discover after our
>>>> +	 * loan request that another page is to be unloaned. However, once
>>>> +	 * the cmm_thread runs again later, this error will automatically
>>>> +	 * be corrected.
>>>> +	 */
>>>> +	if (plpar_page_set_loaned(newpage)) {
>>>> +		/* Unlikely, but possible. Tell the caller not to retry now. */
>>>> +		pr_err_ratelimited("%s: Cannot set page to loaned.", __func__);
>>>> +		return -EBUSY;
>>>> +	}
>>>> +
>>>> +	/* balloon page list reference */
>>>> +	get_page(newpage);
>>>> +
>>>> +	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>>>> +	balloon_page_insert(b_dev_info, newpage);
>>>> +	balloon_page_delete(page);
>>>
>>> I think I am missing a b_dev_info->isolated_pages-- here.
>> 
>> I don't know this code at all, but looking at other balloon drivers they
>> do seem to do that in roughly the same spot.
>> 
>> I'll add it, how can we test that it's correct?
>
> It's certainly correct. We increment when we isolate 
> (balloon_page_isolate()) and decrement when we un-isolate.
>
> Un-isolate happens when we putback a isolated page 
> (balloon_page_putback() - migration aborted) or when we successfully 
> migrate it (via balloon_page_migrate()).
>
> The issue is that we cannot decrement in balloon_page_migrate(), as we 
> have to hold the b_dev_info->pages_lock. That's why we have to do it in 
> the registered callback under lock.

OK, I get it now.

> Please note that b_dev_info->isolated_pages is only needed for a sanity 
> check in balloon_page_dequeue(). That's why I didn't notice during 
> testing. I wonder if we should at some point rip out that sanity check ...

OK. Sanity checks can be good, though checks that call BUG() are less
nice :)  But I'm not an mm expert so I'll defer to you folks on the
sanity check.

For now I've merged this series with the decrement added to
cmm_migratepage().

cheers