Re: [PATCH v1 07/23] mm/balloon_compaction: use a device-independent balloon (list) lock

[David Hildenbrand <david@redhat.com>]

On 21.10.25 22:52, Michael S. Tsirkin wrote:
> On Tue, Oct 21, 2025 at 02:59:12PM +0200, David Hildenbrand wrote:
>> In order to remove the dependency on the page lock for balloon
>> pages, we need a lock that is independent of the page.
>>
>> It's crucial that we can handle the scenario where balloon deflation
>> (clearing page->private) can race with page isolation (using
>> page->private to obtain the balloon_dev_info where the lock currently
>> resides).
>>
>> The current lock in balloon_dev_info is therefore not suitable.
>>
>> Fortunately, we never really have more than a single balloon device
>> per VM, so we can just keep it simple and use a static lock to protect
>> all balloon devices.
>>
>> Based on this change we will remove the dependency on the page lock
>> next.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   include/linux/balloon_compaction.h |  6 ++---
>>   mm/balloon_compaction.c            | 36 +++++++++++++++++-------------
>>   2 files changed, 23 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
>> index 3109d3c43d306..e2d9eb40e1fbb 100644
>> --- a/include/linux/balloon_compaction.h
>> +++ b/include/linux/balloon_compaction.h
>> @@ -21,10 +21,10 @@
>>    *   i. Setting the PG_movable_ops flag and page->private with the following
>>    *	lock order
>>    *	    +-page_lock(page);
>> - *	      +--spin_lock_irq(&b_dev_info->pages_lock);
>> + *	      +--spin_lock_irq(&balloon_pages_lock);
>>    *
>>    *  ii. isolation or dequeueing procedure must remove the page from balloon
>> - *      device page list under b_dev_info->pages_lock.
>> + *      device page list under &balloon_pages_lock
> 
> Using &balloon_pages_lock with an & is kinda weird here.

Indeed, fixed, thanks!

-- 
Cheers

David / dhildenb