Re: [PATCH] mm: Add additional consistency check

[Christoph Lameter]

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.