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

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Message-Id: <201712200233.vBK2X7oX028845@www262.sakura.ne.jp>
Date:   Wed, 20 Dec 2017 11:33:07 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     wei.w.wang@...el.com, 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.xu0@...il.com, nilal@...hat.com, riel@...hat.com
Subject: Re: [PATCH v20 0/7] Virtio-balloon Enhancement

Matthew Wilcox wrote:
> > I think xb_find_set() has a bug in !node path.
> 
> Don't think.  Write a test-case.  Please.  If it shows a bug, then great,

+unsigned long xb_find_set(struct xb *xb, unsigned long size,
+			  unsigned long offset)
+{
+	struct radix_tree_root *root = &xb->xbrt;
+	struct radix_tree_node *node;
+	void __rcu **slot;
+	struct ida_bitmap *bitmap;
+	unsigned long index = offset / IDA_BITMAP_BITS;
+	unsigned long index_end = size / IDA_BITMAP_BITS;
+	unsigned long bit = offset % IDA_BITMAP_BITS;
+
+	if (unlikely(offset >= size))
+		return size;
+
+	while (index <= index_end) {
+		unsigned long ret;
+		unsigned int nbits = size - index * IDA_BITMAP_BITS;
+
+		bitmap = __radix_tree_lookup(root, index, &node, &slot);
+		if (!node) {
+			index = (index | RADIX_TREE_MAP_MASK) + 1;

Why we don't need to reset "bit" to 0 here?
We will continue with wrong offset if "bit != 0", won't we?

+			continue;
+		}
+
+		if (bitmap) {
+			if (nbits > IDA_BITMAP_BITS)
+				nbits = IDA_BITMAP_BITS;
+
+			ret = find_next_bit(bitmap->bitmap, nbits, bit);
+			if (ret != nbits)
+				return ret + index * IDA_BITMAP_BITS;
+		}
+		bit = 0;
+		index++;
+	}
+
+	return size;
+}