Re: [PATCH v4 4/5] mm: Fix page reference leak in soft_offline_page()

[HORIGUCHI NAOYA(堀口 直也)]

On Wed, Jan 13, 2021 at 04:43:32PM -0800, Dan Williams wrote:
> The conversion to move pfn_to_online_page() internal to
> soft_offline_page() missed that the get_user_pages() reference taken by
> the madvise() path needs to be dropped when pfn_to_online_page() fails.
> Note the direct sysfs-path to soft_offline_page() does not perform a
> get_user_pages() lookup.
> 
> When soft_offline_page() is handed a pfn_valid() &&
> !pfn_to_online_page() pfn the kernel hangs at dax-device shutdown due to
> a leaked reference.
> 
> Fixes: feec24a6139d ("mm, soft-offline: convert parameter to pfn")
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Naoya Horiguchi <nao.horiguchi@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

I'm OK if we don't have any other better approach, but the proposed changes
make code a little messy, and I feel that get_user_pages() might be the
right place to fix. Is get_user_pages() expected to return struct page with
holding refcount for offline valid pages?  I thought that such pages are
only used by drivers for dax-devices, but that might be wrong. Can I ask for
a little more explanation from this perspective?

Thanks,
Naoya Horiguchi

> ---
>  mm/memory-failure.c |   20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 5a38e9eade94..78b173c7190c 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1885,6 +1885,12 @@ static int soft_offline_free_page(struct page *page)
>  	return rc;
>  }
>  
> +static void put_ref_page(struct page *page)
> +{
> +	if (page)
> +		put_page(page);
> +}
> +
>  /**
>   * soft_offline_page - Soft offline a page.
>   * @pfn: pfn to soft-offline
> @@ -1910,20 +1916,26 @@ static int soft_offline_free_page(struct page *page)
>  int soft_offline_page(unsigned long pfn, int flags)
>  {
>  	int ret;
> -	struct page *page;
>  	bool try_again = true;
> +	struct page *page, *ref_page = NULL;
> +
> +	WARN_ON_ONCE(!pfn_valid(pfn) && (flags & MF_COUNT_INCREASED));
>  
>  	if (!pfn_valid(pfn))
>  		return -ENXIO;
> +	if (flags & MF_COUNT_INCREASED)
> +		ref_page = pfn_to_page(pfn);
> +
>  	/* Only online pages can be soft-offlined (esp., not ZONE_DEVICE). */
>  	page = pfn_to_online_page(pfn);
> -	if (!page)
> +	if (!page) {
> +		put_ref_page(ref_page);
>  		return -EIO;
> +	}
>  
>  	if (PageHWPoison(page)) {
>  		pr_info("%s: %#lx page already poisoned\n", __func__, pfn);
> -		if (flags & MF_COUNT_INCREASED)
> -			put_page(page);
> +		put_ref_page(ref_page);
>  		return 0;
>  	}
>  
>