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

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Date:   Wed, 20 Dec 2017 16:13:16 +0000
From:   "Wang, Wei W" <wei.w.wang@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
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 Wednesday, December 20, 2017 8:26 PM, Matthew Wilcox wrote:
> On Wed, Dec 20, 2017 at 06:34:36PM +0800, Wei Wang wrote:
> > On 12/19/2017 10:05 PM, Tetsuo Handa wrote:
> > > I think xb_find_set() has a bug in !node path.
> >
> > I think we can probably remove the "!node" path for now. It would be
> > good to get the fundamental part in first, and leave optimization to
> > come as separate patches with corresponding test cases in the future.
> 
> You can't remove the !node path.  You'll see !node when the highest set bit
> is less than 1024.  So do something like this:
> 
> 	unsigned long bit;
> 	xb_preload(GFP_KERNEL);
> 	xb_set_bit(xb, 700);
> 	xb_preload_end();
> 	bit = xb_find_set(xb, ULONG_MAX, 0);
> 	assert(bit == 700);

This above test will result in "!node with bitmap !=NULL", and it goes to the regular "if (bitmap)" path, which finds 700.

A better test would be
...
xb_set_bit(xb, 700);
assert(xb_find_set(xb, ULONG_MAX, 800) == ULONG_MAX);
...

The first try with the "if (bitmap)" path doesn't find a set bit, and the remaining tries will always result in "!node && !bitmap", which implies no set bit anymore and no need to try in this case.

So, I think we can change it to

If (!node && !bitmap)
	return size;


Best,
Wei