linux-kernel - Re: [PATCH v18 05/10] xbitmap: add more operations

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Message-ID: <5A292D94.5000700@intel.com>
Date:   Thu, 07 Dec 2017 20:01:24 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        willy@...radead.org
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,
        liliang.opensource@...il.com, yang.zhang.wz@...il.com,
        quan.xu@...yun.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v18 05/10] xbitmap: add more operations

On 12/03/2017 09:50 AM, Tetsuo Handa wrote:
> Matthew Wilcox wrote:
>> On Fri, Dec 01, 2017 at 03:09:08PM +0000, Wang, Wei W wrote:
>>> On Friday, December 1, 2017 9:02 PM, Tetsuo Handa wrote:
>>>> If start == end is legal,
>>>>
>>>>     for (; start < end; start = (start | (IDA_BITMAP_BITS - 1)) + 1) {
>>>>
>>>> makes this loop do nothing because 10 < 10 is false.
>>> How about "start <= end "?
>> Don't ask Tetsuo for his opinion, write some userspace code that uses it.
>>
> Please be sure to prepare for "end == -1UL" case, for "start < end" will become
> true when "start = (start | (IDA_BITMAP_BITS - 1)) + 1" made "start == 0" due to
> overflow.

I think there is one more corner case with this API: searching for bit 
"1" from [0, ULONG_MAX] while no bit is set in the range, there appear 
to be no possible value that we can return (returning "end + 1" will be 
"ULONG_MAX + 1", which is 0)
I plan to make the "end" be exclusive of the searching, that is, [start, 
end), and return "end" if no such bit is found.

For cases like [16, 16), returning 16 doesn't mean bit 16 is 1 or 0, it 
simply means there is no bits to search in the given range, since 16 is 
exclusive.

Please let me know if you have a different thought.

Best,
Wei