On Wed, Oct 31, 2012 at 9:48 PM, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Thu, 1 Nov 2012, Minchan Kim wrote:
>
>> 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.
>>
>
> Ah, interesting. This is from commit 5515061d22f0 ("mm: throttle direct
> reclaimers if PF_MEMALLOC reserves are low and swap is backed by network
> storage"). Thanks for adding Mel to the cc.
>
> The oom killer specifically has logic for this condition: when calling
> out_of_memory() the first thing it does is
>
> if (fatal_signal_pending(current))
> set_thread_flag(TIF_MEMDIE);
>
> to allow it access to memory reserves so that it may exit if it's having
> trouble. But that ends up never happening because of the above code that
> Minchan has identified.
>
> So we either need to do set_thread_flag(TIF_MEMDIE) in try_to_free_pages()
> as well or revert that early return entirely; there's no justification
> given for it in the comment nor in the commit log. I'd rather remove it
> and allow the oom killer to trigger and grant access to memory reserves
> itself if necessary.
>
> Mel, how does commit 5515061d22f0 deal with threads looping forever if
> they need memory in the exit path since the oom killer never gets called?
>
> That aside, it doesn't seem like this is the issue that Luigi is reporting
> since his patch that avoids deferring the oom killer presumably fixes the
> issue for him. So it turns out the oom killer must be getting called.
>
> Luigi, can you try this instead? It applies to the latest git but should
> be easily modified to apply to any 3.x kernel you're running.
> ---
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -310,26 +310,13 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
> if (!task->mm)
> return OOM_SCAN_CONTINUE;
>
> - if (task->flags & PF_EXITING) {
> + if (task->flags & PF_EXITING && !force_kill) {
> /*
> - * If task is current and is in the process of releasing memory,
> - * allow the "kill" to set TIF_MEMDIE, which will allow it to
> - * access memory reserves. Otherwise, it may stall forever.
> - *
> - * The iteration isn't broken here, however, in case other
> - * threads are found to have already been oom killed.
> + * If this task is not being ptraced on exit, then wait for it
> + * to finish before killing some other task unnecessarily.
> */
> - if (task == current)
> - return OOM_SCAN_SELECT;
> - else if (!force_kill) {
> - /*
> - * If this task is not being ptraced on exit, then wait
> - * for it to finish before killing some other task
> - * unnecessarily.
> - */
> - if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
> - return OOM_SCAN_ABORT;
> - }
> + if (!(task->group_leader->ptrace & PT_TRACE_EXIT))
> + return OOM_SCAN_ABORT;
> }
> return OOM_SCAN_OK;
> }
> @@ -706,11 +693,11 @@ void out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
> return;
>
> /*
> - * If current has a pending SIGKILL, then automatically select it. The
> - * goal is to allow it to allocate so that it may quickly exit and free
> - * its memory.
> + * If current has a pending SIGKILL or is exiting, then automatically
> + * select it. The goal is to allow it to allocate so that it may
> + * quickly exit and free its memory.
> */
> - if (fatal_signal_pending(current)) {
> + if (fatal_signal_pending(current) || current->flags & PF_EXITING) {
> set_thread_flag(TIF_MEMDIE);
> return;
> }
I tested this change with my load and it appears to also prevent the deadlocks.
I have a question though. I thought only one process was allowed to
be in TIF_MEMDIE state, but I don't see anything that prevents this
code (before or after the change) from setting the flag in multiple
processes. Is this a problem?
Thanks!
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