Re: [PATCH] mm: add MM_SWAPENTS and page table when calculate tasksize in lowmem_scan()

["Figo.zhang" <figo1802@gmail.com>]

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2016-02-17 8:35 GMT+08:00 David Rientjes <rientjes@google.com>:

> On Tue, 16 Feb 2016, Greg Kroah-Hartman wrote:
>
> > On Tue, Feb 16, 2016 at 05:37:05PM +0800, Xishi Qiu wrote:
> > > Currently tasksize in lowmem_scan() only calculate rss, and not
> include swap.
> > > But usually smart phones enable zram, so swap space actually use ram.
> >
> > Yes, but does that matter for this type of calculation?  I need an ack
> > from the android team before I could ever take such a core change to
> > this code...
> >
>
> The calculation proposed in this patch is the same as the generic oom
> killer, it's an estimate of the amount of memory that will be freed if it
> is killed and can exit.  This is better than simply get_mm_rss().
>
> However, I think we seriously need to re-consider the implementation of
> the lowmem killer entirely.  It currently abuses the use of TIF_MEMDIE,
> which should ideally only be set for one thread on the system since it
> allows unbounded access to global memory reserves.
>


i don't understand why it need wait 1 second:

if (test_tsk_thread_flag(p, TIF_MEMDIE) &&
   time_before_eq(jiffies, lowmem_deathpending_timeout)) {
task_unlock(p);
rcu_read_unlock();
return 0;                             <= why return rather than continue?
}

and it will retry and wait many CPU times if one task holding the TIF_MEMDI.
   shrink_slab_node()
       while()
           shrinker->scan_objects();
                     lowmem_scan()
                                 if (test_tsk_thread_flag(p, TIF_MEMDIE) &&
                                       time_before_eq(jiffies,
lowmem_deathpending_timeout))



>
> It also abuses the user-visible /proc/self/oom_score_adj tunable: this
> tunable is used by the generic oom killer to bias or discount a proportion
> of memory from a process's usage.  This is the only supported semantic of
> the tunable.  The lowmem killer uses it as a strict prioritization, so any
> process with oom_score_adj higher than another process is preferred for
> kill, REGARDLESS of memory usage.  This leads to priority inversion, the
> user is unable to always define the same process to be killed by the
> generic oom killer and the lowmem killer.  This is what happens when a
> tunable with a very clear and defined purpose is used for other reasons.
>
> I'd seriously consider not accepting any additional hacks on top of this
> code until the implementation is rewritten.
>
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