linux-kernel - Re: [PATCH v19 3/7] xbitmap: add more operations

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Date:   Sat, 16 Dec 2017 18:14:12 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
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, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v19 3/7] xbitmap: add more operations

On 12/15/2017 12:29 AM, Tetsuo Handa wrote:
> Wei Wang wrote:
>> I used the example of xb_clear_bit_range(), and xb_find_next_bit() is
>> the same fundamentally. Please let me know if anywhere still looks fuzzy.
> I don't think it is the same for xb_find_next_bit() with set == 0.
>
> +		if (radix_tree_exception(bmap)) {
> +			unsigned long tmp = (unsigned long)bmap;
> +			unsigned long ebit = bit + 2;
> +
> +			if (ebit >= BITS_PER_LONG)
> +				continue;
> +			if (set)
> +				ret = find_next_bit(&tmp, BITS_PER_LONG, ebit);
> +			else
> +				ret = find_next_zero_bit(&tmp, BITS_PER_LONG,
> +							 ebit);
> +			if (ret < BITS_PER_LONG)
> +				return ret - 2 + IDA_BITMAP_BITS * index;
>
> What I'm saying is that find_next_zero_bit() will not be called if you do
> "if (ebit >= BITS_PER_LONG) continue;" before calling find_next_zero_bit().
>
> When scanning "0000000000000000000000000000000000000000000000000000000000000001",
> "bit < BITS_PER_LONG - 2" case finds "0" in this word but
> "bit >= BITS_PER_LONG - 2" case finds "0" in next word or segment.
>
> I can't understand why this is correct behavior. It is too much puzzling.
>

OK, I'll post out a version without the exceptional path.

Best,
Wei