Re: [patch 1/2] mm, memcg: avoid oom notification when current needs access to memory reserves

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Nov 27, 2013 at 04:22:18PM -0800, David Rientjes wrote:
> On Wed, 27 Nov 2013, Johannes Weiner wrote:
> 
> > > The patch is drawing the line at "the kernel can no longer do anything to 
> > > free memory", and that's the line where userspace should be notified or a 
> > > process killed by the kernel.
> > >
> > > Giving current access to memory reserves in the oom killer is an
> > > optimization so that all reclaim is exhausted prior to declaring
> > > that they are necessary, the kernel still has the ability to allow
> > > that process to exit and free memory.
> > 
> > "they" are necessary?
> > 
> 
> Memory reserves.
> 
> > > This is the same as the oom notifiers within the kernel that free
> > > memory from s390 and powerpc archs: the kernel still has the ability
> > > to free memory.
> > 
> > They're not the same at all.  One is the kernel freeing memory, the
> > other is a random coincidence.
> > 
> 
> Current is on the way to memory freeing because it has a pending SIGKILL 
> or is already exiting, it simply needs access to memory reserves to do so.  
> This was originally introduced to prevent the oom killer from having to 
> scan the set of eligible processes and silently giving it access to memory 
> reserves; we didn't want to emit all of the messages to the kernel log 
> because scripts (and admins) were looking at the kernel log and seeing 
> that the oom killer killed something when it really came from a different 
> source or was already exiting.
> 
> We have a differing opinion on what to consider the point of oom (the 
> "notification line that has to be drawn").  My position is to notify 
> userspace when the kernel has exhausted its capability to free memory 
> without killing something.  In the case of current exiting or having a 
> pending SIGKILL, memory is going to be freed, the oom killer simply needs 
> to preempt the tasklist scan.  The situation is going to be remedied.  I 
> defined the notification with this patch to only happen when the kernel 
> can't free any memory without a kill so that userspace may do so itself.  
> Michal concurred with that position.

The long-standing, user-visible definition of the current line agrees
with me.  You can't just redefine this, period.

I tried to explain to you how insane the motivation for this patch is,
but it does not look like you are reading what I write.  But you don't
get to change user-visible behavior just like that anyway, much less
so without a sane reason, so this was a complete waste of time :-(

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