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Subject: Re: [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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