On Fri, Nov 30, 2012 at 11:55:08AM -0600, Mark Tinguely wrote:
> On 11/30/12 10:09, Christoph Hellwig wrote:
> >> use_alloc_page:
> >>- start = BBTOB(bp->b_map.bm_bn)>> PAGE_SHIFT;
> >>- end = (BBTOB(bp->b_map.bm_bn + bp->b_length) + PAGE_SIZE - 1)
> >>+ start = BBTOB(bp->b_maps.bm_bn)>> PAGE_SHIFT;
> >>+ end = (BBTOB(bp->b_maps.bm_bn + bp->b_length) + PAGE_SIZE - 1)
> >Btw, how is this logic supposed to work for discontiguous buffers?
> >Each of them might straddle boundaries individually, so doing the
> >start/end calculation for the number of pages isn't going to be correct.
> eeek. yep, I will loop through and count the pages needed in each
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....