Re: [PATCH v2 1/3] xfs: use b_maps[] for discontiguous buffers

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Sat, Dec 01, 2012 at 09:21:10AM +1100, Dave Chinner wrote:
> > eeek. yep, I will loop through and count the pages needed in each
> > segment.
> 
> That's wrong - the pages must be mappable as a contiguous memory
> range. That's how this code avoids copying the data from
> discontiguous page ranges into a contiguous mapped memory range.
> If you separate each IO into to it's own set of pages, then you have
> to completely rewrite _xfs_buf_ioapply(), which AFAICT works just
> fine with the page allocation that is done right now....

Yeah, you're right.