Re: Oops in a driver while using SLUB as a SLAB allocator

[Nicolas Ferre]

Hugh Dickins :
> On Sun, 24 Jun 2007, Russell King wrote:
> > On Sun, Jun 24, 2007 at 11:24:16AM +0100, Hugh Dickins wrote:
> > > On Sun, 24 Jun 2007, Russell King wrote:
> > > > On Fri, Jun 22, 2007 at 07:39:33PM +0100, Hugh Dickins wrote:
> > > >
> > > > Please forward the original problem report.
> > > Done.
> > Okay, that seems to back up my suspicions - it's definitely AT91-based.
> > Since AT91-based machines do not have a DMA coherent cache,
> > arch_is_coherent() must be defined to '0'.  The only way that kmalloc
> > could be reached is if that were defined to something other than '0',
> > and if that's done on a machine with DMA incoherent caches, that will
> > lead to data corruption.
>
> Yes, having looked through that now, I agree with you 100%.

AT91 _is_ defined with the arch_is_coherent() switch set to 0 (in 
include/asm-arm/memory.h and not overloaded).

> > I think we need to wait for Nicolas to respond on this issue before
> > running headlong into applying a sticky plaster for something which is
> > actually a deeper issue.
>
> No need for Nicolas to respond, I think I've found what's "wrong":
> see below.
>
> > However, the arch_is_coherent() path _is_ buggy as it stands, but in
> > more than the way identified thus far.  Eg, it doesn't set __GFP_DMA
> > appropriately for various DMA masks, so it might return DMA inaccessible
> > memory.

Ok it is bad but, in AT91, all memory is accessible with DMA.

[..]

> The slub allocation which gives rise to Nicolas' oops in not in
> ARM, but (I'm guessing) in drivers/mmc/core/sd.c: one of those
> 	status = kmalloc(64, GFP_KERNEL);
> where status is passed down for the response from mmc_sd_switch.

True, the oops appears after a mmc switch command (#6 command).

[..]

Not sure I can add much to what Hugh has said. If you need some more 
precision anyway, I will try to answer.

Regards,
--
Nicolas Ferre


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