Re: Calling free_pages on part of the memory returned by get_free_pages?

[Andi Kleen <andi@firstfloor.org>]

Timur Tabi <timur@freescale.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