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SubjectRe: [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG
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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