Messages in this thread | | | Date | Tue, 10 Apr 2012 23:52:05 +0400 | From | Konstantin Khlebnikov <> | Subject | Re: [PATCH v2] mm: correctly synchronize rss-counters at exit/exec |
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Andrew Morton wrote: > On Tue, 10 Apr 2012 21:07:32 +0400 > Konstantin Khlebnikov<khlebnikov@openvz.org> 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.
But it works as designed =)
> > > Do we think we should backport this into -stable kernels? How hard is > it to make that warning come out? >
Warning was introduced in v3.3-3720-gc3f0327, so no releases with them. And I think older kernels can live fine with slightly racy rss counters.
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