On Wed, Jan 13, 2021 at 5:50 PM HORIGUCHI NAOYA(堀口 直也) naoya.horiguchi@nec.com wrote:
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@linux-foundation.org Cc: Naoya Horiguchi nao.horiguchi@gmail.com Cc: Michal Hocko mhocko@kernel.org Reviewed-by: David Hildenbrand david@redhat.com Reviewed-by: Oscar Salvador osalvador@suse.de Cc: stable@vger.kernel.org Signed-off-by: Dan Williams dan.j.williams@intel.com
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?
The motivation for ZONE_DEVICE is to allow get_user_pages() for "offline" pfns.
soft_offline_page() wants to only operate on "online" pfns.
get_user_pages() on a dax-mapping returns an "offline" ZONE_DEVICE page.
When pfn_to_online_page() fails the get_user_pages() reference needs to be dropped.
To be honest I dislike the entire flags based scheme for communicating the fact that page reference obtained by madvise needs to be dropped later. I'd rather pass a non-NULL 'struct page *' than redo pfn_to_page() conversions in the leaf functions, but that's a much larger change.