On Wed, Oct 30, 2019 at 11:24:09AM +0100, Michal Hocko wrote: > 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@xxxxxxxxxxxxxx> > > > Suggested-by: Minchan Kim <minchan@xxxxxxxxxx> > > > Cc: Michal Hocko <mhocko@xxxxxxxx> > > Acked-by: Minchan Kim <minchan@xxxxxxxxxx> > > > > -stable material from v4.15. > > Which commit does this fix? Fixes: tag would be highly appreciated. Oops, I missed to mention it. Fixes: aa8d22a11da9 (mm: swap: SWP_SYNCHRONOUS_IO: skip swapcache only if swapped page has no other reference) > > -- > Michal Hocko > SUSE Labs
