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Date: Tue, 10 Oct 2017 22:09:43 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: wei.w.wang@...el.com, mst@...hat.com
Cc: virtio-dev@...ts.oasis-open.org, linux-kernel@...r.kernel.org,
qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
kvm@...r.kernel.org, linux-mm@...ck.org, mhocko@...nel.org,
akpm@...ux-foundation.org, mawilcox@...rosoft.com,
david@...hat.com, cornelia.huck@...ibm.com,
mgorman@...hsingularity.net, aarcange@...hat.com,
amit.shah@...hat.com, pbonzini@...hat.com, willy@...radead.org,
liliang.opensource@...il.com, yang.zhang.wz@...il.com,
quan.xu@...yun.com
Subject: Re: [PATCH v16 3/5] virtio-balloon: VIRTIO_BALLOON_F_SG
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
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.
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