linux-kernel - Re: [PATCH] mm: Add additional consistency check

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Date:   Tue, 11 Apr 2017 13:03:01 -0500 (CDT)
From:   Christoph Lameter <cl@...ux.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Add additional consistency check

On Tue, 11 Apr 2017, Michal Hocko wrote:

> >
> > There is a flag SLAB_DEBUG_OBJECTS that is available for this check.
>
> Which is way too late, at least for the kfree path. page->slab_cache
> on anything else than PageSlab is just a garbage. And my understanding
> of the patch objective is to stop those from happening.

We are looking here at SLAB. SLUB code can legitimately have a compound
page there because large allocations fallback to the page allocator.

Garbage would be attempting to free a page that has !PageSLAB set but also
is no compound page. That condition is already checked in kfree() with a
BUG_ON() and that BUG_ON has been there for a long time. Certainly we can
make SLAB consistent if there is no check there already. Slab just
attempts a free on that object which will fail too.

So we are already handling that condition. Why change things? Add a BUG_ON
if you want to make SLAB consistent.