Re: [BUG] slab debug vs. L1 alignement

[Manfred Spraul]

Benjamin Herrenschmidt wrote:

> > > Yes, I understand that, but that is wrong for GFP_DMA imho. Also, 
> > > SLAB_MUST_HWCACHE_ALIGN just disables redzoning, which is not smart,
> > > I'd rather allocate more and keep both redzoning and cache alignement,
> > > that would help catch some of those subtle problems when a chip DMA
> > > engine plays funny tricks.
> > >
> > >      
> > I don't want to upgrade SLAB_HWCACHE_ALIGN to SLAB_MUST_HWCACHE_ALIGN 
> > depending on GFP_DMA: IIRC one arch (ppc64?) marks everything as 
> > GFP_DMA, because all memory is DMA capable.
> >    
>
> Same for ppc32. Anyway, I don't like MUST_HWCACHE_ALIGN because it
> just disables redzoning, I'd rather allocate more and do both redzoning
> and cache alignement.
>  
I have a patch that creates helper functions that make that simple. The 
patch is stuck right now, because it exposes a bug in the i386 debug 
register handling. I'll add it redzoning with MUST_HWCACHE_ALIGN after 
that one is in.

> Anyway, I _still_ think it's stupid to return non-aligned buffers, both
> for performances, and because that prevents from dealing with such cases,
> typically the SCSI layer assumes alignement here among others...
>  
I don't care about performance with slab debugging on. kmalloc(4096,) 
usually takes ~40 cpu ticks on i386. With debugging on, it includes a 
memset and an open coded memcmp - my guess is a few thousand cpu ticks, 
and that's intentional. Do not enable it on production systems.

> Regarding O_DIRECT with an unaligned pointer, I haven't looked at this
> case yet, I suppose it will be broken in a whole lot of cases.
>  
Hmm. That means slab debugging did it's job: The driver contains the 
wrong assumption that all pointers are cache line aligned. Without slab 
debugging, this would result in rare data corruptions in O_DIRECT apps. 
With slab debugging on, it's exposed immediately.

--
   Manfred

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