linux-kernel - Re: [PATCH v16 1/5] lib/xbitmap: Introduce xbitmap

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Message-ID: <3b575060-e30b-1afe-96a8-6a9f732d5528@I-love.SAKURA.ne.jp>
Date:   Mon, 9 Oct 2017 20:30:00 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Wei Wang <wei.w.wang@...el.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, mst@...hat.com,
        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 1/5] lib/xbitmap: Introduce xbitmap

On 2017/09/30 13:05, Wei Wang wrote:
>  /**
> + *  xb_preload - preload for xb_set_bit()
> + *  @gfp_mask: allocation mask to use for preloading
> + *
> + * Preallocate memory to use for the next call to xb_set_bit(). This function
> + * returns with preemption disabled. It will be enabled by xb_preload_end().
> + */
> +void xb_preload(gfp_t gfp)
> +{
> +	if (__radix_tree_preload(gfp, XB_PRELOAD_SIZE) < 0)
> +		preempt_disable();
> +
> +	if (!this_cpu_read(ida_bitmap)) {
> +		struct ida_bitmap *bitmap = kmalloc(sizeof(*bitmap), gfp);
> +
> +		if (!bitmap)
> +			return;
> +		bitmap = this_cpu_cmpxchg(ida_bitmap, NULL, bitmap);
> +		kfree(bitmap);
> +	}
> +}

I'm not sure whether this function is safe.

__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...