linux-kernel - Re: [PATCH] vmalloc: back off only when the current task is OOM killed

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Date:   Tue, 10 Oct 2017 16:17:33 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     hannes@...xchg.org, akpm@...ux-foundation.org,
        alan@...yncelyn.cymru, hch@....de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] vmalloc: back off only when the current task is OOM
 killed

On Tue 10-10-17 23:13:21, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 10-10-17 21:47:02, Tetsuo Handa wrote:
> > > I think that massive vmalloc() consumers should be (as well as massive
> > > alloc_page() consumers) careful such that they will be chosen as first OOM
> > > victim, for vmalloc() does not abort as soon as an OOM occurs.
> > 
> > No. This would require to spread those checks all over the place. That
> > is why we have that logic inside the allocator which fails the
> > allocation at certain point in time. Large/unbound/user controlled sized
> > allocations from the kernel are always a bug and really hard one to
> > protect from. It is simply impossible to know the intention.
> > 
> > > Thus, I used
> > > set_current_oom_origin()/clear_current_oom_origin() when I demonstrated
> > > "complete" depletion.
> > 
> > which was a completely artificial example as already mentioned.
> > 
> > > > I have tried to explain this is not really needed before but you keep
> > > > insisting which is highly annoying. The patch as is is not harmful but
> > > > it is simply _pointless_ IMHO.
> > > 
> > > Then, how can massive vmalloc() consumers become careful?
> > > Explicitly use __vmalloc() and pass __GFP_NOMEMALLOC ?
> > > Then, what about adding some comment like "Never try to allocate large
> > > memory using plain vmalloc(). Use __vmalloc() with __GFP_NOMEMALLOC." ?
> > 
> > Come on! Seriously we do expect some competence from the code running in
> > the kernel space. We do not really need to add a comment that you
> > shouldn't shoot your head because it might hurt. Please try to focus on
> > real issues. There are many of them to chase after...
> > 
> My understanding is that vmalloc() is provided for allocating large memory
> where kmalloc() is difficult to satisfy. If we say "do not allocate large
> memory with vmalloc() because large allocations from the kernel are always
> a bug", it sounds like denial of raison d'etre of vmalloc(). Strange...

try to find some middle ground between literal following the wording and
a common sense. In kernel anything larger than order-3 is a large
allocation. The large we are arguing here is MBs of memory.

-- 
Michal Hocko
SUSE Labs