linux-kernel - Re: [BUG 2.6.24-rc3-git6] SLUB's ksize() fails for size

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Date:	Sun, 2 Dec 2007 17:43:09 +0100
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Tetsuo Handa" <penguin-kernel@...ove.sakura.ne.jp>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG 2.6.24-rc3-git6] SLUB's ksize() fails for size > 2048.

On Dec 2, 2007 5:30 PM, Vegard Nossum <vegard.nossum@...il.com> wrote:
> On Dec 2, 2007 11:39 AM, Tetsuo Handa
> <penguin-kernel@...ove.sakura.ne.jp> wrote:
> > Hello.
> >
> > I can't pass memory allocated by kmalloc() to ksize()
> > if it is allocated by SLUB allocator and
> > size is larger than (I guess) PAGE_SIZE / 2.
> >
> > Regards.
> The error of ksize() seems to be that it does not check if the
> allocation was made by SLUB or the page allocator. Maybe something
> like this will fix it? (completely untested)

That didn't work. I guess that's what you get for no testing ;-) After
some more investigations, it seems that this is the correct way to fix
it (and tested!):

diff --git a/mm/slub.c b/mm/slub.c
index 9acb413..b9f37cb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2558,8 +2558,12 @@ size_t ksize(const void *object)
        if (unlikely(object == ZERO_SIZE_PTR))
                return 0;

-       page = get_object_page(object);
+       page = virt_to_head_page(object);
        BUG_ON(!page);
+
+       if (unlikely(!PageSlab(page)))
+               return PAGE_SIZE << compound_order(page);
+
        s = page->slab;
        BUG_ON(!s);


> It's going to round up, though, so you would get ksize(kmalloc(2049))
> = PAGE_SIZE.


Vegard
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