On Mon, 9 Apr 2012, KOSAKI Motohiro wrote: > On Mon, Apr 9, 2012 at 4:03 PM, Konstantin Khlebnikov > <khlebnikov@xxxxxxxxxx> wrote: > > On task's exit do_exit() calls sync_mm_rss() but this is not enough, > > there can be page-faults after this point, for example exit_mm() -> > > mm_release() -> put_user() (for processing tsk->clear_child_tid). > > Thus there may be some rss-counters delta in current->rss_stat. > > Seems reasonable. Yes, I think Konstantin has probably caught it; but I'd like to hear confirmation from Markus. > but I have another question. Do we have any reason to > keep sync_mm_rss() in do_exit()? I havn't seen any reason that thread exiting > makes rss consistency. IIRC it's all about the hiwater_rss/maxrss stuff: we want to sync the maximum rss into mm->hiwater_rss before it's transferred to signal->maxrss, and later made visible to the user though getrusage(RUSAGE_CHILDREN,) - does your reading confirm that? Konstantin now finds the child_tid and futex stuff can trigger faults raising rss beyond that point, but usually it won't go higher than when it was captured for maxrss there. The sync_mm_rss() added by this patch (after "tsk->mm = NULL" so *_mm_counter_fast() cannot store any more into the tsk even if there were more faults) is solely to satisfy Konstantin's check_mm(), and it is irritating to have that duplicated on the exit path. I'd be happy to see the new one put under CONFIG_DEBUG_VM along with check_mm(), once it's had a few -rcs of exposure without. Hugh > > > > > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx> > > Reported-by: Markus Trippelsdorf <markus@xxxxxxxxxxxxxxx> > > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > > --- > > kernel/exit.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/kernel/exit.c b/kernel/exit.c > > index d8bd3b42..8e09dbe 100644 > > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -683,6 +683,7 @@ static void exit_mm(struct task_struct * tsk) > > enter_lazy_tlb(mm, current); > > task_unlock(tsk); > > mm_update_next_owner(mm); > > + sync_mm_rss(mm); > > mmput(mm); > > } > >