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.