Re: zram OOM behavior

[David Rientjes <rientjes@google.com>]

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 
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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