Re: WARNING in kvmalloc_node

[Jason Wang]

On 2018å02æ14æ 20:29, Jesper Dangaard Brouer wrote:
> 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.

I see verifier guarantees this.

>   Thus, there isn't any
> reason for GFP_ATOMIC here.

Want me to send a patch to remove GFP_ATOMIC here?

Thanks