Re: [PATCH] mm: do not free shared swap slots

[Michal Hocko <mhocko@kernel.org>]

On Tue 29-10-19 17:24:47, Minchan Kim wrote:
> On Tue, Oct 22, 2019 at 04:51:34PM +0530, Vinayak Menon wrote:
> > The following race is observed due to which a processes faulting
> > on a swap entry, finds the page neither in swapcache nor swap. This
> > causes zram to give a zero filled page that gets mapped to the
> > process, resulting in a user space crash later.
> > 
> > Consider parent and child processes Pa and Pb sharing the same swap
> > slot with swap_count 2. Swap is on zram with SWP_SYNCHRONOUS_IO set.
> > Virtual address 'VA' of Pa and Pb points to the shared swap entry.
> > 
> > Pa                                       Pb
> > 
> > fault on VA                              fault on VA
> > do_swap_page                             do_swap_page
> > lookup_swap_cache fails                  lookup_swap_cache fails
> >                                          Pb scheduled out
> > swapin_readahead (deletes zram entry)
> > swap_free (makes swap_count 1)
> >                                          Pb scheduled in
> >                                          swap_readpage (swap_count == 1)
> >                                          Takes SWP_SYNCHRONOUS_IO path
> >                                          zram enrty absent
> >                                          zram gives a zero filled page
> > 
> > Fix this by making sure that swap slot is freed only when swap count
> > drops down to one.
> > 
> > Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
> > Suggested-by: Minchan Kim <minchan@google.com>
> > Cc: Michal Hocko <mhocko@suse.com>
> Acked-by: Minchan Kim <minchan@kernel.org>
> 
> -stable material from v4.15.

Which commit does this fix? Fixes: tag would be highly appreciated.

-- 
Michal Hocko
SUSE Labs