linux-kernel - Re: possible bug in page allocation mechanism

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Message-Id: <20070215215350.4dbd04ca.akpm@linux-foundation.org>
Date:	Thu, 15 Feb 2007 21:53:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tim Cullen <timcullen2001@...oo.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: possible bug in page allocation mechanism

On Thu, 15 Feb 2007 14:11:42 -0800 (PST) Tim Cullen <timcullen2001@...oo.com> wrote:

> There appears to be a inconsistenancy with reference
> counts on pages allocated with alloc_pages when order
> is greater than zero. In buffered_rmqueue when order
> != 0 then __rmqueue is called. This returns a page
> pointer that is really a pointer to the first page in
> a group of pages. Subsequently prep_new_page is called
> on the first page of the group but not on any others.
> This results in the first page having a reference
> count of 1 while all other pages in the allocation
> have a reference count of 0. I would think that all
> pages in the same allocation should all have the same
> reference count at the end of the allocation.
> 
> I've looked at this in the 2.6.20, 2.6.19.1, and the
> 2.6.17.7 kernels. They contain the same code in this
> area.
> 

That's as-designed.  All page refcount manipulations on a higher-order page
are supposed to be against the head pageframe.

Also, we have the compound page logic in there to force code which tries to
manipulate the refcount of a tail page to be redirected to the head page.

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