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

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Date:   Thu, 14 Dec 2017 19:47:29 +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: [virtio-dev] Re: [PATCH v19 3/7] xbitmap: add more operations

On 12/14/2017 11:47 AM, Wei Wang wrote:
> On 12/13/2017 10:16 PM, Tetsuo Handa wrote:
>
>
>>
>>>                           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_start;
>>>                   } else if (bitmap) {
>>>                           if (set)
>>>                                   ret = find_next_bit(bitmap->bitmap,
>>> IDA_BITMAP_BITS, bit);
>>>                           else
>>>                                   ret = 
>>> find_next_zero_bit(bitmap->bitmap,
>>> IDA_BITMAP_BITS, bit);
>> "bit" may not be 0 for the first round and "bit" is always 0 afterwords.
>> But where is the guaranteed that "end" is a multiple of 
>> IDA_BITMAP_BITS ?
>> Please explain why it is correct to use IDA_BITMAP_BITS unconditionally
>> for the last round.
>
> There missed something here, it will be:
>
> nbits = min(end - ida_start + 1, IDA_BITMAP_BITS - bit);


captured a bug here, should be:
nbits = min(end - ida_start + 1, (unsigned long)IDA_BITMAP_BITS);


> if (set)
>     ret = find_next_bit(bitmap->bitmap, nbits, bit);
> else
>     ret = find_next_zero_bit(bitmap->bitmap,
>                                            nbits, bit);
> if (ret < nbits)
>     return ret + ida_start;
>
>

Best,
Wei