linux-kernel - Re: [PATCH v2] mm/balloon_compaction: avoid duplicate page removal

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Date:   Thu, 18 Jul 2019 08:26:11 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, xdeguillard@...are.com, namit@...are.com,
        akpm@...ux-foundation.org, pagupta@...hat.com, riel@...riel.com,
        dave.hansen@...el.com, david@...hat.com, konrad.wilk@...cle.com,
        yang.zhang.wz@...il.com, nitesh@...hat.com, lcapitulino@...hat.com,
        aarcange@...hat.com, pbonzini@...hat.com,
        alexander.h.duyck@...ux.intel.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2] mm/balloon_compaction: avoid duplicate page removal

On Thu, Jul 18, 2019 at 05:27:20PM +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.
> This is necessary when it's used from balloon_page_enqueue_list, but
> not from balloon_page_enqueue_one.
> 
> So remove the list_del balloon_page_enqueue_one, and update some
> comments as a reminder.
> 
> Signed-off-by: Wei Wang <wei.w.wang@...el.com>


ok I posted v3 with typo fixes. 1/2 is this patch with comment changes. Pls take a look.

> ---
> ChangeLong:
> v1->v2: updated some comments
> 
>  mm/balloon_compaction.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
> index 83a7b61..8639bfc 100644
> --- a/mm/balloon_compaction.c
> +++ b/mm/balloon_compaction.c
> @@ -21,7 +21,6 @@ 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);
> @@ -33,7 +32,7 @@ static void balloon_page_enqueue_one(struct balloon_dev_info *b_dev_info,
>   * @b_dev_info: balloon device descriptor where we will insert a new page to
>   * @pages: pages to enqueue - allocated using balloon_page_alloc.
>   *
> - * Driver must call it to properly enqueue a balloon pages before definitively
> + * Driver must call it to properly enqueue balloon pages before definitively
>   * removing it from the guest system.
>   *
>   * Return: number of pages that were enqueued.
> @@ -47,6 +46,7 @@ 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++;
>  	}
> @@ -128,13 +128,19 @@ struct page *balloon_page_alloc(void)
>  EXPORT_SYMBOL_GPL(balloon_page_alloc);
>  
>  /*
> - * balloon_page_enqueue - allocates a new page and inserts it into the balloon
> - *			  page list.
> + * balloon_page_enqueue - inserts a new page into the balloon page list.
> + *
>   * @b_dev_info: balloon device descriptor where we will insert a new page to
>   * @page: new page to enqueue - allocated using balloon_page_alloc.
>   *
>   * Driver must call it to properly enqueue a new allocated balloon page
>   * before definitively removing it from the guest system.
> + *
> + * Drivers must not call balloon_page_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.
> + *
>   * This function returns the page address for the recently enqueued page or
>   * NULL in the case we fail to allocate a new page this turn.
>   */
> -- 
> 2.7.4