linux-kernel - Re: [PATCH] nommu: fix kobjsize() for SLOB and SLUB

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Date:	Sun, 1 Jun 2008 11:22:04 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Paul Mundt <lethal@...ux-sh.org>,
	David Howells <dhowells@...hat.com>,
	Christoph Lameter <clameter@....com>,
	LKML <linux-kernel@...r.kernel.org>, cooloney@...nel.org,
	akpm@...ux-foundation.org, mpm@...enic.com
Subject: Re: [PATCH] nommu: fix kobjsize() for SLOB and SLUB

On Sun, 1 Jun 2008, Pekka Enberg wrote:
> > Not until the page->index bits are killed, otherwise you aren't fixing
> > anything. SLOB on nommu with those page->index tests will automatically
> > oops today, before or after your patches. Until that's resolved, there's
> > no point in pretending like kobjsize() has been "fixed". As no one has
> > come up with a valid reason for those tests existing in the first place,
> > simply having your patches and killing the BUG_ON()'s seems ok.
> 
> Sorry if I'm starting to sound like a broken record, but can you explain why
> removing the ->index bits are safe? I mean, if removing them is really okay,
> that means we don't hit that code path with SLAB at all?

Paul, so with something like this, the WARN_ON never triggers?

diff --git a/mm/nommu.c b/mm/nommu.c
index dca93fc..38eec2e 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -109,16 +109,23 @@ unsigned int kobjsize(const void *objp)
 	 * If the object we have should not have ksize performed on it,
 	 * return size of 0
 	 */
-	if (!objp || (unsigned long)objp >= memory_end || !((page = virt_to_page(objp))))
+	if (!objp)
+		return 0;
+
+	if ((unsigned long)objp >= memory_end)
+		return 0;
+
+	page = virt_to_page(objp);
+	if (!page)
 		return 0;
 
 	if (PageSlab(page))
 		return ksize(objp);
 
-	BUG_ON(page->index < 0);
-	BUG_ON(page->index >= MAX_ORDER);
+	if (WARN_ON(!PageCompound(page)))
+		return 0;
 
-	return (PAGE_SIZE << page->index);
+	return PAGE_SIZE << compound_order(page);
 }
 
 /*
--
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