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@xxxxxxxxxxxxxx> > Suggested-by: Minchan Kim <minchan@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Minchan Kim <minchan@xxxxxxxxxx> -stable material from v4.15. Thanks.
