On Thu, Jan 20, 2011 at 12:33:46PM +1100, Dave Chinner wrote:
> It's case b) that I'm mainly worried about, esp. w.r.t the 64k page
> size on ia64/ppc. If we only track a single dirty bit in the page,
> then every sub-page, non-appending write to an uncached region of a
> file becomes a RMW cycle to initialise the areas around the write
> correctly. The question is whether we care about this enough given
> that we return at least PAGE_SIZE in stat() to tell applications the
> optimal IO size to avoid RMW cycles.
Note that this generally is only true for the first write into the
region - after that we'll have the rest read into the cache. But
we also have the same issue for appending writes if they aren't
> And if we only do IO on whole pages (i.e regardless of block size)
> .writepage suddenly becomes a lot simpler, as well as being trivial
> to implement our own .readpage/.readpages....
I don't think it simplifies writepage a lot. All the buffer head
handling goes away, but we'll still need to do xfs_bmapi calls at
block size granularity. Why would you want to replaced the
readpage/readpages code? The generic mpage helpers for it do just fine.