Re: [BUG] slab debug vs. L1 alignement

[Manfred Spraul <manfred@colorfullife.com>]

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