linux-kernel - Re: [PATCH v20 0/7] Virtio-balloon Enhancement

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Date:   Thu, 21 Dec 2017 04:14:37 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "david@...hat.com" <david@...hat.com>,
        "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "aarcange@...hat.com" <aarcange@...hat.com>,
        "amit.shah@...hat.com" <amit.shah@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "liliang.opensource@...il.com" <liliang.opensource@...il.com>,
        "yang.zhang.wz@...il.com" <yang.zhang.wz@...il.com>,
        "quan.xu0@...il.com" <quan.xu0@...il.com>,
        "nilal@...hat.com" <nilal@...hat.com>,
        "riel@...hat.com" <riel@...hat.com>
Subject: Re: [PATCH v20 0/7] Virtio-balloon Enhancement

On Thu, Dec 21, 2017 at 10:49:44AM +0800, Wei Wang wrote:
> On 12/21/2017 01:10 AM, Matthew Wilcox wrote:
> One more question is about the return value, why would it be ambiguous? I
> think it is the same as find_next_bit() which returns the found bit or size
> if not found.

Because find_next_bit doesn't reasonably support a bitmap which is
ULONG_MAX in size.  The point of XBitmap is to support a bitmap which
is ULONG_MAX in size, so every possible return value is a legitimate
"we found a bit here".  There's no value which can possibly be used for
"no bit was found".