On Thu, 2007-08-09 at 12:27 -0700, Christoph Lameter wrote:
> On Thu, 9 Aug 2007, Peter Zijlstra wrote:
> 
> > Less conditionals. We already have a branch for mapping, why create
> > another?
> 
> Ah. Okay. This also avoids an interrupt enable disable since you can use 
> __ functions. Hmmm... Would be good if we could move the vmstat 
> NR_WRITEBACK update there too. Can a page without a mapping be under 
> writeback? (Direct I/O?)

DIO still uses the mapping afaik (it needs to invalidate the page before
and after the OP).

But you could put the increment in both paths, and use the irq disable
from the mapping branch - which should be the most frequent case anyway.