Re: [Qemu-devel] [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG

[Wei Wang <wei.w.wang@intel.com>]

On 10/10/2017 09:09 PM, Tetsuo Handa wrote:
> Wei Wang wrote:
>>> And even if we could remove balloon_lock, you still cannot use
>>> __GFP_DIRECT_RECLAIM at xb_set_page(). I think you will need to use
>>> "whether it is safe to wait" flag from
>>> "[PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify()" .
>> Without the lock being held, why couldn't we use __GFP_DIRECT_RECLAIM at
>> xb_set_page()?
> Because of dependency shown below.
>
> leak_balloon()
>    xb_set_page()
>      xb_preload(GFP_KERNEL)
>        kmalloc(GFP_KERNEL)
>          __alloc_pages_may_oom()
>            Takes oom_lock
>            out_of_memory()
>              blocking_notifier_call_chain()
>                leak_balloon()
>                  xb_set_page()
>                    xb_preload(GFP_KERNEL)
>                      kmalloc(GFP_KERNEL)
>                        __alloc_pages_may_oom()
>                          Fails to take oom_lock and loop forever

__alloc_pages_may_oom() uses mutex_trylock(&oom_lock).

I think the second __alloc_pages_may_oom() will not continue since the
first one is in progress.

>
> By the way, is xb_set_page() safe?
> Sleeping in the kernel with preemption disabled is a bug, isn't it?
> __radix_tree_preload() returns 0 with preemption disabled upon success.
> xb_preload() disables preemption if __radix_tree_preload() fails.
> Then, kmalloc() is called with preemption disabled, isn't it?
> But xb_set_page() calls xb_preload(GFP_KERNEL) which might sleep with
> preemption disabled.

Yes, I think that should not be expected, thanks.

I plan to change it like this:

bool xb_preload(gfp_t gfp)
{
         if (!this_cpu_read(ida_bitmap)) {
                 struct ida_bitmap *bitmap = kmalloc(sizeof(*bitmap), gfp);

                 if (!bitmap)
                         return false;
                 bitmap = this_cpu_cmpxchg(ida_bitmap, NULL, bitmap);
                 kfree(bitmap);
         }

         if (__radix_tree_preload(gfp, XB_PRELOAD_SIZE) < 0)
                 return false;

         return true;
}


Best,
Wei