Re: [PATCH v1 13/29] mm/balloon_compaction: stop using __ClearPageMovable()

[David Hildenbrand <david@redhat.com>]

On 01.07.25 12:03, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:54PM +0200, David Hildenbrand wrote:
>> We can just look at the balloon device (stored in page->private), to see
>> if the page is still part of the balloon.
>>
>> As isolated balloon pages cannot get released (they are taken off the
>> balloon list while isolated), we don't have to worry about this case in
>> the putback and migration callback. Add a WARN_ON_ONCE for now.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Seems reasonable, one comment below re: comment.
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
>> ---
>>   include/linux/balloon_compaction.h |  4 +---
>>   mm/balloon_compaction.c            | 11 +++++++++++
>>   2 files changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
>> index bfc6e50bd004b..9bce8e9f5018c 100644
>> --- a/include/linux/balloon_compaction.h
>> +++ b/include/linux/balloon_compaction.h
>> @@ -136,10 +136,8 @@ static inline gfp_t balloon_mapping_gfp_mask(void)
>>    */
>>   static inline void balloon_page_finalize(struct page *page)
>>   {
>> -	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION)) {
>> -		__ClearPageMovable(page);
>> +	if (IS_ENABLED(CONFIG_BALLOON_COMPACTION))
>>   		set_page_private(page, 0);
>> -	}
>>   	/* PageOffline is sticky until the page is freed to the buddy. */
>>   }
>>
>> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
>> index ec176bdb8a78b..e4f1a122d786b 100644
>> --- a/mm/balloon_compaction.c
>> +++ b/mm/balloon_compaction.c
>> @@ -206,6 +206,9 @@ static bool balloon_page_isolate(struct page *page, isolate_mode_t mode)
>>   	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
>>   	unsigned long flags;
>>
>> +	if (!b_dev_info)
>> +		return false;
>> +
>>   	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>>   	list_del(&page->lru);
>>   	b_dev_info->isolated_pages++;
>> @@ -219,6 +222,10 @@ static void balloon_page_putback(struct page *page)
>>   	struct balloon_dev_info *b_dev_info = balloon_page_device(page);
>>   	unsigned long flags;
>>
>> +	/* Isolated balloon pages cannot get deflated. */
>> +	if (WARN_ON_ONCE(!b_dev_info))
>> +		return;
>> +
>>   	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>>   	list_add(&page->lru, &b_dev_info->pages);
>>   	b_dev_info->isolated_pages--;
>> @@ -234,6 +241,10 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
>>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
>>   	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>>
>> +	/* Isolated balloon pages cannot get deflated. */
> 
> Hm do you mean migrated?

Well, they can get migrated, obviously :)

Deflation would be the other code path where we would remove a balloon 
page from the balloon, and invalidate page->private, suddenly seeing 
!b_dev_info here.

But that cannot happen, as isolation takes them off the balloon list. So 
deflating the balloon cannot find them until un-isolated.

-- 
Cheers,

David / dhildenb