linux-kernel - Re: [PATCH 3/6] struct page: add field for vm

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Date:   Fri, 2 Feb 2018 12:43:25 -0600 (CST)
From:   Christopher Lameter <cl@...ux.com>
To:     Igor Stoppa <igor.stoppa@...wei.com>
cc:     jglisse@...hat.com, keescook@...omium.org, mhocko@...nel.org,
        labbott@...hat.com, hch@...radead.org, willy@...radead.org,
        linux-security-module@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 3/6] struct page: add field for vm_struct

On Thu, 1 Feb 2018, Igor Stoppa wrote:

> > Would it not be better to use compound page allocations here?
> > page_head(whatever) gets you the head page where you can store all sorts
> > of information about the chunk of memory.
>
> Can you please point me to this function/macro? I don't seem to be able
> to find it, at least not in 4.15

Ok its compound_head(). See also the use in the SLAB and SLUB allocator.

> During hardened user copy permission check, I need to confirm if the
> memory range that would be exposed to userspace is a legitimate
> sub-range of a pmalloc allocation.

If you save the size in the head page struct then you could do that pretty
fast.

> I cannot comment on your proposal because I do not know where to find
> the reference you made, or maybe I do not understand what you mean :-(

compund pages are higher order pages that are handled as a single page by
the VM. See https://lwn.net/Articles/619514/