Messages in this thread | | | Date | Wed, 11 Oct 2017 09:51:46 +0800 | From | Wei Wang <> | Subject | Re: [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG |
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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
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