Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap read fails

["ying.huang@intel.com" <ying.huang@intel.com>]

On Mon, 2022-04-25 at 09:49 +0200, David Hildenbrand wrote:
> On 25.04.22 09:41, ying.huang@intel.com wrote:
> > Hi, Miaohe,
> > 
> > On Sun, 2022-04-24 at 17:11 +0800, Miaohe Lin wrote:
> > > There is a bug in unuse_pte(): when swap page happens to be unreadable,
> > > page filled with random data is mapped into user address space.  In case
> > > of error, a special swap entry indicating swap read fails is set to the
> > > page table.  So the swapcache page can be freed and the user won't end up
> > > with a permanently mounted swap because a sector is bad.  And if the page
> > > is accessed later, the user process will be killed so that corrupted data
> > > is never consumed.  On the other hand, if the page is never accessed, the
> > > user won't even notice it.
> > > 
> > > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > > Acked-by: David Hildenbrand <david@redhat.com>
> > > ---
> > >  include/linux/swap.h    |  7 ++++++-
> > >  include/linux/swapops.h | 10 ++++++++++
> > >  mm/memory.c             |  5 ++++-
> > >  mm/swapfile.c           | 11 +++++++++++
> > >  4 files changed, 31 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > > index 5553189d0215..b82c196d8867 100644
> > > --- a/include/linux/swap.h
> > > +++ b/include/linux/swap.h
> > > @@ -55,6 +55,10 @@ static inline int current_is_kswapd(void)
> > >   * actions on faults.
> > >   */
> > > 
> > > +#define SWP_SWAPIN_ERROR_NUM 1
> > > +#define SWP_SWAPIN_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
> > > +			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
> > > +			     SWP_PTE_MARKER_NUM)
> > > 
> > > 
> > 
> > It appears wasteful to use another swap device number. 
> 
> Do we really care?
> 
> We currently use 5 bits for swap types, so we have a total of 32.
> 
> SWP_HWPOISON_NUM -> 1
> SWP_MIGRATION_NUM -> 3
> SWP_PTE_MARKER_NUM -> 1
> SWP_DEVICE_NUM -> 4
> SWP_SWAPIN_ERROR_NUM -> 1
> 
> Which would leave us with 32 - 10 = 22 swap devices. IMHO that's plenty
> for real life scenarios.

Creating multiple swap partitions on one disk can improve the
scalability of swap subsystem, although we usually don't have so many
disks for swap. 

> I'd prefer reworking this when we really run into trouble (and we could
> think about using more bits for applicable architectures then, for
> select 64bit architectures it might be fairly easily possible).

Best Regards,
Huang, Ying