On 03/13, Linus Torvalds wrote:
>
> On Mar 13, 2014 8:11 AM, "Oleg Nesterov" <o...@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.
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