Re: + mm-fix-unexpected-zeroed-page-mapping-with-zram-swap.patch added to -mm tree

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (22/03/30 14:02), Sergey Senozhatsky wrote:
> On (22/03/25 20:43), Andrew Morton wrote:
> > Two processes under CLONE_VM cloning, user process can be corrupted by
> > seeing zeroed page unexpectedly.
> > 
> >     CPU A                        CPU B
> > 
> > do_swap_page                do_swap_page
> > SWP_SYNCHRONOUS_IO path     SWP_SYNCHRONOUS_IO path
> > swap_readpage valid data
> >   swap_slot_free_notify
> >     delete zram entry
> >                             swap_readpage zeroed(invalid) data
> >                             pte_lock
> >                             map the *zero data* to userspace
> >                             pte_unlock
> > pte_lock
> > if (!pte_same)
> >   goto out_nomap;
> > pte_unlock
> > return and next refault will
> > read zeroed data
> > 
> > The swap_slot_free_notify is bogus for CLONE_VM case since it doesn't
> > increase the refcount of swap slot at copy_mm so it couldn't catch up
> > whether it's safe or not to discard data from backing device.  In the
> > case, only the lock it could rely on to synchronize swap slot freeing is
> > page table lock.  Thus, this patch gets rid of the swap_slot_free_notify
> > function.  With this patch, CPU A will see correct data.
> > 
> >     CPU A                        CPU B
> > 
> > do_swap_page                do_swap_page
> > SWP_SYNCHRONOUS_IO path     SWP_SYNCHRONOUS_IO path
> >                             swap_readpage original data
> >                             pte_lock
> >                             map the original data
> >                             swap_free
> >                               swap_range_free
> >                                 bd_disk->fops->swap_slot_free_notify
> > swap_readpage read zeroed data
> >                             pte_unlock
> > pte_lock
> > if (!pte_same)
> >   goto out_nomap;
> > pte_unlock
> > return
> > on next refault will see mapped data by CPU B
> > 
> > The concern of the patch would increase memory consumption since it could
> > keep wasted memory with compressed form in zram as well as uncompressed
> > form in address space.  However, most of cases of zram uses no readahead
> > and do_swap_page is followed by swap_free so it will free the compressed
> > form from in zram quickly.
> 
> Minchan, a quick question, shouldn't this instead revert 3f2b1a04f4493?

Never mind! My bad.

The patch looks good to me.