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Date: Thu, 13 Mar 2014 17:36:32 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Davidlohr Bueso <davi@redhat.com>, Davidlohr Bueso <davidlohr@hp.com>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH -next] mm,vmacache: also flush cache for VM_CLONE
Message-ID: <20140313163632.GA30737@redhat.com>
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On 03/13, Linus Torvalds wrote:
>
> On Mar 13, 2014 8:11 AM, "Oleg Nesterov" <oleg@redhat.com> wrote:
> >
> > Suppose that a kernel thread T does, say,
> >
> >         use_mm(foreign_mm);
> >         get_user(...);
> >         unuse_mm();
>
> That would be a major bug. Kernel threads cannot access use memory.

Unless a kernel thread does use_mm() ;)

> Has
> somebody added anything that crazy?

Hmm. aio no longer uses use_mm()... But there are other users:

	drivers/usb/gadget/inode.c  582   use_mm(mm);
	drivers/vhost/vhost.c       211   use_mm(dev->mm);
	virt/kvm/async_pf.c          68   use_mm(mm);

And yes, they do copy_to/from_user().

Hmm, but at first glance async_pf_execute() doesn't need use_mm() at all.
And perhaps other callers can use get_user_pages() too.

> The kernel thread "use_mm" is to avoid unnecessary context switches of the
> tlb when switching to a kennel thread, exactly *because* a kernel thread is
> never supposed to access use space, so it cannot care what user memory is
> attached.

It seems that you are talking about switch_mm-like things or I misunderstood.
use_mm() actually changes ->mm, not only ->active_mm.

> So I object vet much to making kernel threads special on this context,
> unless the "special" bit is some VM_BUG_ON() or similar.

See above. Perhaps we can kill use_mm() (personally I don't think we should),
but until then vmacache needs this check afaics.

Oleg.