Re: [PATCH v1] mm/balloon_compaction: avoid duplicate page removal

[Wei Wang <wei.w.wang@intel.com>]

On 07/18/2019 12:31 PM, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 10:23:30AM +0800, Wei Wang wrote:
>> Fixes: 418a3ab1e778 (mm/balloon_compaction: List interfaces)
>>
>> A #GP is reported in the guest when requesting balloon inflation via
>> virtio-balloon. The reason is that the virtio-balloon driver has
>> removed the page from its internal page list (via balloon_page_pop),
>> but balloon_page_enqueue_one also calls "list_del"  to do the removal.
> I would add here "this is necessary when it's used from
> balloon_page_enqueue_list but not when it's called
> from balloon_page_enqueue".
>
>> So remove the list_del in balloon_page_enqueue_one, and have the callers
>> do the page removal from their own page lists.
>>
>> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> Patch is good but comments need some work.
>
>> ---
>>   mm/balloon_compaction.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
>> index 83a7b61..1a5ddc4 100644
>> --- a/mm/balloon_compaction.c
>> +++ b/mm/balloon_compaction.c
>> @@ -11,6 +11,7 @@
>>   #include <linux/export.h>
>>   #include <linux/balloon_compaction.h>
>>   
>> +/* Callers ensure that @page has been removed from its original list. */
> This comment does not make sense. E.g. balloon_page_enqueue
> does nothing to ensure this. And drivers are not supposed
> to care how the page lists are managed. Pls drop.
>
> Instead please add the following to balloon_page_enqueue:
>
>
> 	Note: drivers must not call balloon_page_list_enqueue on

Probably, you meant balloon_page_enqueue here.

The description for balloon_page_enqueue also seems incorrect:
"allocates a new page and inserts it into the balloon page list."
This function doesn't do any allocation itself.
Plan to reword it: inserts a new page into the balloon page list."

Best,
Wei