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Date:   Thu, 7 Dec 2017 17:41:18 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        willy@...radead.org, 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, 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 Thu, Dec 07, 2017 at 08:01:24PM +0800, Wei Wang wrote:
> 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

Matthew is right though - you want to include tests for all
these corner cases.

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