Re: [PATCH] scsi: align shost->hostdata to cacheline

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Fri, 2008-02-15 at 07:49 +0900, Tejun Heo wrote:
> James Bottomley wrote:
> > On Thu, 2008-02-14 at 18:48 +0900, Tejun Heo wrote:
> >> shost->hostdata can contain arbitrary data including DMA target
> >> buffers.  Align it to cacheline.
> >>
> >> Signed-off-by: Tejun Heo <htejun@gmail.com>
> >> ---
> >> James, what do you think?
> > 
> > Hmm, it will blow out the host size ... although that's not such a huge
> > problem since there are relatively few of them in most running kernels.
> > What's the actual use case for this, though?  The host structure is
> > allocated in ordinary memory ... we don't make sure it's DMAable, and
> > most HBAs that want to use memory for mailboxes need coherent memory
> > anyway.
> 
> "As it can contain arbitrary structure, it should follow the largest
> meaningful alignment to allow the contained structure proper alignment."
> is the logic.  I think it's generally RTTD for inline private data but
> feel free to disagree.

Well, the way we usually do that is to have the host float the alignment
if necessary.  The problem with relying on __cacheline_aligned for an
allocated structure is that it only works if the allocated structure
actually begins on a cacheline.  Kmalloc (which is where we get the host
from) doesn't necessarily obey this if certain slab debugging flags are
present.

James