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Date:   Tue, 24 May 2022 19:35:52 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     zhenwei pi <pizhenwei@bytedance.com>
Cc:     akpm@linux-foundation.org, naoya.horiguchi@nec.com, mst@redhat.com,
        david@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jasowang@redhat.com, virtualization@lists.linux-foundation.org,
        pbonzini@redhat.com, peterx@redhat.com, qemu-devel@nongnu.org
Subject: Re: [PATCH 3/3] virtio_balloon: Introduce memory recover
Message-ID: <Yo0zmP28FqpivlxF@google.com>
References: <20220520070648.1794132-1-pizhenwei@bytedance.com>
 <20220520070648.1794132-4-pizhenwei@bytedance.com>
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On Fri, May 20, 2022, zhenwei pi wrote:
> @@ -59,6 +60,12 @@ enum virtio_balloon_config_read {
>  	VIRTIO_BALLOON_CONFIG_READ_CMD_ID = 0,
>  };
>  
> +/* the request body to commucate with host side */
> +struct __virtio_balloon_recover {
> +	struct virtio_balloon_recover vbr;
> +	__virtio32 pfns[VIRTIO_BALLOON_PAGES_PER_PAGE];

I assume this is copied from virtio_balloon.pfns, which also uses __virtio32, but
isn't that horribly broken?  PFNs are 'unsigned long', i.e. 64 bits on 64-bit kernels.
x86-64 at least most definitely generates 64-bit PFNs.  Unless there's magic I'm
missing, page_to_balloon_pfn() will truncate PFNs and feed the host bad info.

> @@ -494,6 +511,198 @@ static void update_balloon_size_func(struct work_struct *work)
>  		queue_work(system_freezable_wq, work);
>  }
>  
> +/*
> + * virtballoon_memory_failure - notified by memory failure, try to fix the
> + *                              corrupted page.
> + * The memory failure notifier is designed to call back when the kernel handled
> + * successfully only, WARN_ON_ONCE on the unlikely condition to find out any
> + * error(memory error handling is a best effort, not 100% coverd).
> + */
> +static int virtballoon_memory_failure(struct notifier_block *notifier,
> +				      unsigned long pfn, void *parm)
> +{
> +	struct virtio_balloon *vb = container_of(notifier, struct virtio_balloon,
> +						 memory_failure_nb);
> +	struct page *page;
> +	struct __virtio_balloon_recover *out_vbr;
> +	struct scatterlist sg;
> +	unsigned long flags;
> +	int err;
> +
> +	page = pfn_to_online_page(pfn);
> +	if (WARN_ON_ONCE(!page))
> +		return NOTIFY_DONE;
> +
> +	if (PageHuge(page))
> +		return NOTIFY_DONE;
> +
> +	if (WARN_ON_ONCE(!PageHWPoison(page)))
> +		return NOTIFY_DONE;
> +
> +	if (WARN_ON_ONCE(page_count(page) != 1))
> +		return NOTIFY_DONE;
> +
> +	get_page(page); /* balloon reference */
> +
> +	out_vbr = kzalloc(sizeof(*out_vbr), GFP_KERNEL);
> +	if (WARN_ON_ONCE(!out_vbr))
> +		return NOTIFY_BAD;

Not that it truly matters, but won't failure at this point leak the poisoned page?