Re: zram OOM behavior

[Minchan Kim <minchan@kernel.org>]

On Wed, Oct 31, 2012 at 11:54:07AM -0700, David Rientjes wrote:
> On Wed, 31 Oct 2012, Minchan Kim wrote:
> 
> > It sounds right in your kernel but principal problem is min_filelist_kbytes patch.
> > If normal exited process in exit path requires a page and there is no free page
> > any more, it ends up going to OOM path after try to reclaim memory several time.
> > Then,
> > In select_bad_process,
> > 
> >         if (task->flags & PF_EXITING) {
> >                if (task == current)             <== true
> >                         return OOM_SCAN_SELECT;
> > In oom_kill_process,
> > 
> >         if (p->flags & PF_EXITING)
> >                 set_tsk_thread_flag(p, TIF_MEMDIE);
> > 
> > At last, normal exited process would get a free page.
> > 
> 
> select_bad_process() won't actually select the process for oom kill, 
> though, if there are other PF_EXITING threads other than current.  So if 
> multiple threads are page faulting on tsk->robust_list, then no thread 
> ends up getting killed.  The temporary workaround would be to do a kill -9 
> so that the logic in out_of_memory() could immediately give such threads 
> access to memory reserves so the page fault will succeed.  The real fix 

It's not true any more.
3.6 includes following code in try_to_free_pages

        /*   
         * Do not enter reclaim if fatal signal is pending. 1 is returned so
         * that the page allocator does not consider triggering OOM
         */
        if (fatal_signal_pending(current))
                return 1;

So the hunged task never go to the OOM path and could be looping forever.

> would be to audit all possible cases in between setting 
> tsk->flags |= PF_EXITING and tsk->mm = NULL that could cause a memory 
> allocation and make exemptions for them in oom_scan_process_thread().
> 
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-- 
Kind regards,
Minchan Kim

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