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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Subject: Re: mm: memory-hotplug: enable memory hotplug to handle hugepage
Date: Tue, 12 May 2015 09:04:55 +0000
Message-ID: <20150512090454.GD3068@hori1.linux.bs1.fc.nec.co.jp>
References: <20150511111748.GA20660@mwanda>
 <20150511235443.GA8513@hori1.linux.bs1.fc.nec.co.jp>
 <20150512084339.GN16501@mwanda>
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List-ID: <linux-mm.kvack.org>
To: Dan Carpenter <dan.carpenter@oracle.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>

On Tue, May 12, 2015 at 11:43:39AM +0300, Dan Carpenter wrote:
> On Mon, May 11, 2015 at 11:54:44PM +0000, Naoya Horiguchi wrote:
> > @@ -1086,7 +1086,8 @@ static void dissolve_free_huge_page(struct page *=
page)
> >   */
> >  void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long e=
nd_pfn)
> >  {
> > -	unsigned int order =3D 8 * sizeof(void *);
> > +	/* Initialized to "high enough" value which is capped later */
> > +	unsigned int order =3D 8 * sizeof(void *) - 1;
>=20
> Why not use UINT_MAX?  It's more clear that it's not valid that way.

It's OK if code checker doesn't show "too much right shift" warning.
With UINT_MAX, inserting VM_BUG_ON(order =3D=3D UINT_MAX) after for_each_hs=
tate
loop might be safer (1 << UINT_MAX is clearly wrong.)

> Otherwise doing a complicated calculation it makes it seem like we will
> use the variable.

OK.

Is the below OK for you?
---
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 271e4432734c..804437505a84 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1188,7 +1188,7 @@ static void dissolve_free_huge_page(struct page *page=
)
  */
 void dissolve_free_huge_pages(unsigned long start_pfn, unsigned long end_p=
fn)
 {
-	unsigned int order =3D 8 * sizeof(void *);
+	unsigned int order =3D UINT_MAX;
 	unsigned long pfn;
 	struct hstate *h;
=20
@@ -1200,6 +1200,7 @@ void dissolve_free_huge_pages(unsigned long start_pfn=
, unsigned long end_pfn)
 		if (order > huge_page_order(h))
 			order =3D huge_page_order(h);
 	VM_BUG_ON(!IS_ALIGNED(start_pfn, 1 << order));
+	VM_BUG_ON(order =3D=3D UINT_MAX);
 	for (pfn =3D start_pfn; pfn < end_pfn; pfn +=3D 1 << order)
 		dissolve_free_huge_page(pfn_to_page(pfn));
 }=

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