Re: [PATCH v1 02/29] mm/balloon_compaction: convert balloon_page_delete() to balloon_page_finalize()

[David Hildenbrand <david@redhat.com>]

On 30.06.25 17:15, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:43PM +0200, David Hildenbrand wrote:
>> Let's move the removal of the page from the balloon list into the single
>> caller, to remove the dependency on the PG_isolated flag and clarify
>> locking requirements.
>>
>> We'll shuffle the operations a bit such that they logically make more sense
>> (e.g., remove from the list before clearing flags).
>>
>> In balloon migration functions we can now move the balloon_page_finalize()
>> out of the balloon lock and perform the finalization just before dropping
>> the balloon reference.
>>
>> Document that the page lock is currently required when modifying the
>> movability aspects of a page; hopefully we can soon decouple this from the
>> page lock.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   arch/powerpc/platforms/pseries/cmm.c |  2 +-
>>   drivers/misc/vmw_balloon.c           |  3 +-
>>   drivers/virtio/virtio_balloon.c      |  4 +--
>>   include/linux/balloon_compaction.h   | 43 +++++++++++-----------------
>>   mm/balloon_compaction.c              |  3 +-
>>   5 files changed, 21 insertions(+), 34 deletions(-)
>>
>> diff --git a/arch/powerpc/platforms/pseries/cmm.c b/arch/powerpc/platforms/pseries/cmm.c
>> index 5f4037c1d7fe8..5e0a718d1be7b 100644
>> --- a/arch/powerpc/platforms/pseries/cmm.c
>> +++ b/arch/powerpc/platforms/pseries/cmm.c
>> @@ -532,7 +532,6 @@ static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
>>
>>   	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>>   	balloon_page_insert(b_dev_info, newpage);
>> -	balloon_page_delete(page);
> 

Hi Lorenzo,

as always, thanks for the detailed review!

> We seem to just be removing this and not replacing with finalize, is this right?

See below.

> 
>>   	b_dev_info->isolated_pages--;
>>   	spin_unlock_irqrestore(&b_dev_info->pages_lock, flags);
>>
>> @@ -542,6 +541,7 @@ static int cmm_migratepage(struct balloon_dev_info *b_dev_info,
>>   	 */
>>   	plpar_page_set_active(page);
>>
>> +	balloon_page_finalize(page);

^ here it is, next to the put_page() just like for the other cases.

Or did you mean something else?

>>   	/* balloon page list reference */
>>   	put_page(page);
>>
>> diff --git a/drivers/misc/vmw_balloon.c b/drivers/misc/vmw_balloon.c
>> index c817d8c216413..6653fc53c951c 100644
>> --- a/drivers/misc/vmw_balloon.c
>> +++ b/drivers/misc/vmw_balloon.c
>> @@ -1778,8 +1778,7 @@ static int vmballoon_migratepage(struct balloon_dev_info *b_dev_info,
>>   	 * @pages_lock . We keep holding @comm_lock since we will need it in a
>>   	 * second.
>>   	 */
>> -	balloon_page_delete(page);
>> -
>> +	balloon_page_finalize(page);
>>   	put_page(page);
>>


[...]

>> -/*
>> - * balloon_page_delete - delete a page from balloon's page list and clear
>> - *			 the page->private assignement accordingly.
>> - * @page    : page to be released from balloon's page list
>> - *
>> - * Caller must ensure the page is locked and the spin_lock protecting balloon
>> - * pages list is held before deleting a page from the balloon device.
>> - */
>> -static inline void balloon_page_delete(struct page *page)
>> -{
>> -	__ClearPageOffline(page);
>> -	__ClearPageMovable(page);
>> -	set_page_private(page, 0);
>> -	/*
>> -	 * No touch page.lru field once @page has been isolated
>> -	 * because VM is using the field.
>> -	 */
>> -	if (!PageIsolated(page))
>> -		list_del(&page->lru);
> 
> I don't see this check elsewhere, is it because, as per the 1/xx of this series,
> because by the time we do the finalize

balloon_page_delete() was used on two paths

1) Removing a page from the balloon for deflation through 
balloon_page_list_dequeue()

2) Removing an isolated page from the balloon for migration in the 
per-driver migration handlers. Isolated pages were already removed from 
the balloon list during ... isolation.

With this change, 1) does the list_del(&page->lru) manually and 2) only 
calls balloon_page_finalize().

During 1) the same reasoning as in 1/xx applies: isolated pages cannot 
be in the balloon list.

> 
>> -}
>> -
>>   /*
>>    * balloon_page_device - get the b_dev_info descriptor for the balloon device
>>    *			 that enqueues the given page.
>> @@ -141,12 +120,6 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
>>   	list_add(&page->lru, &balloon->pages);
>>   }
>>
>> -static inline void balloon_page_delete(struct page *page)
>> -{
>> -	__ClearPageOffline(page);
>> -	list_del(&page->lru);
>> -}
>> -
>>   static inline gfp_t balloon_mapping_gfp_mask(void)
>>   {
>>   	return GFP_HIGHUSER;
>> @@ -154,6 +127,22 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
>>
>>   #endif /* CONFIG_BALLOON_COMPACTION */
>>
>> +/*
>> + * balloon_page_finalize - prepare a balloon page that was removed from the
>> + *			   balloon list for release to the page allocator
>> + * @page: page to be released to the page allocator
>> + *
>> + * Caller must ensure that the page is locked.
> 
> Can we assert this?

We could, but I'm planning on removing the page lock next (see patch 
description), so not too keen to create more code around that.

Maybe mention that the balloon lock should not be held?

Not a limitation. It could be called with it, just not a requirement today.

I suspect that once we remove the page lock, that we might use the 
balloon lock and rework balloon_page_migrate() to take the lock. TBD.

 > >> + */
>> +static inline void balloon_page_finalize(struct page *page)
>> +{
>> +	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION)) {
>> +		__ClearPageMovable(page);
>> +		set_page_private(page, 0);
>> +	}
> 
> Why do we check this? Is this function called from anywhere where that config won't be set?

Sure. balloon_page_list_dequeue() is called from balloon_page_dequeue(), 
which resides outside the CONFIG_BALLOON_COMPACTION ifdef in 
mm/balloon_compaction.c.

At some point (not in this series) we should probably rename

balloon_compaction.c -> balloon.c

To match CONFIG_MEMORY_BALLOON.

Because the compaction part is just one extra bit in there. (an 
important one, but still, you can use the balloon infrastructure without 
compaction/page migration)

-- 
Cheers,

David / dhildenb