Re: WARNING in kvmalloc_node

[Jesper Dangaard Brouer]

On Wed, 14 Feb 2018 13:17:18 +0100
Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:

> On 02/14/2018 01:02 PM, Jason Wang wrote:
> > On 2018å02æ14æ 19:51, Michal Hocko wrote:  
> >> On Wed 14-02-18 19:47:30, Jason Wang wrote:  
> >>> On 2018å02æ14æ 17:28, Daniel Borkmann wrote:  
> >>>> [ +Jason, +Jesper ]
> >>>>
> >>>> On 02/14/2018 09:43 AM, Michal Hocko wrote:  
> >>>>> On Tue 13-02-18 18:55:33, Matthew Wilcox wrote:  
> >>>>>> On Tue, Feb 13, 2018 at 03:59:01PM -0800, syzbot wrote:  
> >>>>> [...]  
> >>>>>>> ÂÂ kvmalloc include/linux/mm.h:541 [inline]
> >>>>>>> ÂÂ kvmalloc_array include/linux/mm.h:557 [inline]
> >>>>>>> ÂÂ __ptr_ring_init_queue_alloc include/linux/ptr_ring.h:474 [inline]
> >>>>>>> ÂÂ ptr_ring_init include/linux/ptr_ring.h:492 [inline]
> >>>>>>> ÂÂ __cpu_map_entry_alloc kernel/bpf/cpumap.c:359 [inline]
> >>>>>>> ÂÂ cpu_map_update_elem+0x3c3/0x8e0 kernel/bpf/cpumap.c:490
> >>>>>>> ÂÂ map_update_elem kernel/bpf/syscall.c:698 [inline]  
> >>>>>> Blame the BPF people, not the MM people ;-)  
> >>>> Heh, not really. ;-)
> >>>>  
> >>>>> Yes. kvmalloc (the vmalloc part) doesn't support GFP_ATOMIC semantic.  
> >>>> Agree, that doesn't work.
> >>>>
> >>>> Bug was added in commit 0bf7800f1799 ("ptr_ring: try vmalloc() when kmalloc() fails").
> >>>>
> >>>> Jason, please take a look at fixing this, thanks!  
> >>> It looks to me the only solution is to revert that commit.  
> >> Do you really need this to be GFP_ATOMIC? I can see some callers are
> >> under RCU read lock but can we perhaps do the allocation outside of this
> >> section?  
> > 
> > If I understand the code correctly, the code would be called by XDP program (usually run inside a bh) which makes it hard to do this.
> > 
> > Rethink of this, we can probably test gfp and not call kvmalloc if GFP_ATOMIC is set in __ptr_ring_init_queue_alloc().  
> 
> That would be one option indeed (probably useful in any case to make the API
> more robust). Another one is to just not use GFP_ATOMIC in cpumap. Looking at
> it, update can neither be called out of a BPF prog since prevented by verifier
> nor under RCU reader side when updating this type of map from syscall path.
> Jesper, any concrete reason we still need GFP_ATOMIC here?

Allocations in cpumap (related to ptr_ring) should only be possible to
be initiated through userspace via bpf-syscall. Thus, there isn't any
reason for GFP_ATOMIC here.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer