linux-kernel - Re: Calling free_pages on part of the memory returned by get_free

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Message-ID: <87r6cvxdd4.fsf@basil.nowhere.org>
Date:	Thu, 24 Apr 2008 21:27:19 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Timur Tabi <timur@...escale.com>
Cc:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: Calling free_pages on part of the memory returned by get_free_pages?

Timur Tabi <timur@...escale.com> writes:

> According to LDD3, if I call get_free_pages() to allocate X bytes, I have to
> free all of those pages with free_pages().  The VM internals are a little bit
> over my head, but I looked at the code and I didn't see why that is a requirement.
>
> For example, let's say I want to allocated 6MB of physically-contiguous memory.
>  If I call x = get_free_pages(11) to get 8MB.  What happens if I then do
> "free_pages(x + 6 * 1024 * 1024, 9)"?
>
> I remember doing this on the 2.4 kernel, and it never gave me any problems.

It is ok, as long as you don't use compound pages (__GFP_COMP) and call
split_page() to fix up the reference counts.

Also you do this to save memory right? The large system hash code does it too
and I used to do it in some 2.4 change with an alloc_pages_exact()
which never made it into 2.6.

If it's reasonably common we should re-add alloc/get_pages_exact() helper to 
make this pattern clear and easier to use.

-Andi
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