On Tue, 10 Apr 2012 21:07:32 +0400 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx> wrote: > mm->rss_stat counters have per-task delta: task->rss_stat, before changing > task->mm pointer kernel must flush this delta with help of sync_mm_rss(). > > do_exit() already calls sync_mm_rss() to flush rss-counters before commiting > rss-statistics into task->signal->maxrss, taskstats, audit and other stuff. > Unfortunately kernel do this before calling mm_relese(), which can call put_user() > for processing task->clear_child_tid. So at this point we can trigger page-faults > and task->rss_stat becomes non-zero again, as result mm->rss_stat becomes > inconsistent and check_mm() will print something like this: > > | BUG: Bad rss-counter state mm:ffff88020813c380 idx:1 val:-1 > | BUG: Bad rss-counter state mm:ffff88020813c380 idx:2 val:1 > > This patch moves sync_mm_rss() into mm_release(), and moves mm_release() out of > do_exit() and calls it earlier. After mm_release() there should be no page-faults. > > ... > > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -751,6 +751,14 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm) > } > tsk->clear_child_tid = NULL; > } > + > + /* > + * Final rss-counter synchronization. After this point must be > + * no page-faults into this mm from current context, otherwise > + * mm->rss_stat will be inconsistent. > + */ > + if (mm) > + sync_mm_rss(mm); > } > Well that's scary. AFACIT `mm' can indeed be NULL here, when a kernel thread calls do_exit(). No implementation of deactivate_mm() actually uses its `mm' arg and I guess that kernel threads never set tsk->clear_child_tid. Whee. Do we think we should backport this into -stable kernels? How hard is it to make that warning come out? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>