Re: build error: sparsemem + SLOB

Yasunori Goto
2006-11-21T05:57:23Z

> 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.

Ok. It's simple. This is fix.

----

This is to disallow to make SLOB with SMP or SPARSEMEM.
This avoids latent troubles of SLOB with SLAB_DESTROY_BY_RCU.
And fix compile error.

This patch is for 2.6.19-rc5-mm2.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>

----
init/Kconfig | 2 +-
1 files changed, 1 insertion(+), 1 deletion(-)

Index: 19-rc5-mm2/init/Kconfig
===================================================================
--- 19-rc5-mm2.orig/init/Kconfig 2006-11-21 13:41:31.000000000 +0900
+++ 19-rc5-mm2/init/Kconfig 2006-11-21 14:21:31.000000000 +0900
@@ -465,7 +465,7 @@ config SHMEM

config SLAB
default y
- bool "Use full SLAB allocator" if EMBEDDED
+ bool "Use full SLAB allocator" if (EMBEDDED && !SMP && !SPARSEMEM)
help
Disabling this replaces the advanced SLAB allocator and
kmalloc support with the drastically simpler SLOB allocator.

--
Yasunori Goto