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 10:18:09PM -0800, Dan Williams wrote:
> On Wed, Jan 13, 2021 at 5:50 PM HORIGUCHI NAOYA(堀口　直也)
> <naoya.horiguchi@xxxxxxx> 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@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?
> 
> The motivation for ZONE_DEVICE is to allow get_user_pages() for "offline" pfns.

I missed this point, thank you.

> 
> soft_offline_page() wants to only operate on "online" pfns.

Right.

> 
> get_user_pages() on a dax-mapping returns an "offline" ZONE_DEVICE page.

OK.

> 
> When pfn_to_online_page() fails the get_user_pages() reference needs
> to be dropped.

The background is clear to me, and I agree with this patch.

Reviewed-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>

> 
> 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.

As Oscar mentions in another email, removing MF_COUNT_INCREASED was
recently discussed and rejected. I think that if pfn-page conversion
layer is somehow factored out from soft/hard offline handler, code would
be more readable/maintainable.

Thanks,
Naoya Horiguchi