Re: build error: sparsemem + SLOB

Matt Mackall
2006-11-20T18:36:32Z

On Mon, Nov 20, 2006 at 05:28:24PM +0000, Hugh Dickins wrote:
> On Mon, 20 Nov 2006, Christoph Lameter wrote:
> >
> > As far as I can tell SLOB is fundamentally racy since it does not support
> > SLAB_DESTROY_BY_RCU correctly. F.e. The constructor for the anon_vma will
> > be called on alloc without regard for RCU, we free an item and reuse it
> > without regard to RCU. This can potentially mess up the anon_vma locking
> > state while we access it.
>
> Good find!
>
> > Is SLOB used at all or have we been lucky so far?
>
> Lucky so far. Well, we'd actually have to be quite unlucky to ever
> see what page_lock_anon_vma/SLAB_DESTROY_BY_RCU are guarding against.
>
> But you're absolutely right that users should not be exposed to such
> unsafety. I'd say SLOB should be disallowed if SMP.

SLOB is an O(N) allocator and is pretty poorly suited to running on
anything like a modern desktop. Disallowing if SMP is probably
reasonable, as even machines with multicore ARM or MIPS will probably
have enough memory to make SLOB a bit painful.

--
Mathematics is the supreme nostalgia of our time.