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

["Michael S. Tsirkin" <mst@redhat.com>]

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
	pages that have been pushed to a list with balloon_page_push
	before removing them with balloon_page_pop.
	To all pages on a list, use balloon_page_list_enqueue instead.

>  static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
>  				     struct page *page)
>  {
> @@ -21,7 +22,6 @@ static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
>  	 * memory corruption is possible and we should stop execution.
>  	 */
>  	BUG_ON(!trylock_page(page));
> -	list_del(&page->lru);
>  	balloon_page_insert(b_dev_info, page);
>  	unlock_page(page);
>  	__count_vm_event(BALLOON_INFLATE);
> @@ -47,6 +47,7 @@ size_t balloon_page_list_enqueue(struct balloon_dev_info *b_dev_info,
>  
>  	spin_lock_irqsave(&b_dev_info->pages_lock, flags);
>  	list_for_each_entry_safe(page, tmp, pages, lru) {
> +		list_del(&page->lru);
>  		balloon_page_enqueue_one(b_dev_info, page);
>  		n_pages++;
>  	}
> -- 
> 2.7.4