Re: btrfs on bcache

[Kent Overstreet <kmo@daterainc.com>]

On Wed, Jan 08, 2014 at 07:35:32PM +0000, Chris Mason wrote:
> On Mon, 2014-01-06 at 15:37 -0800, Kent Overstreet wrote:
> > Ok, I looked again at the relevant btrfs code, I guess I can see how this printk
> > isn't normally triggered. But Chris, _what on earth_ is btrfs trying to check
> > for here? And why is it using bv_offset and bv_len further down in
> > end_bio_extent_readpage()?
> 
> After the IO is done, we're recording the specific logical byte range
> that covered the IO.  In practice its always the full page, we can
> switch to just trusting PAGE_CACHE_SIZE.

Yeah, the code already assumes it was doing PAGE_CACHE_SIZE reads; what
you're effectively checking is that the driver did the bvec all at once,
and that it didn't process half a bvec, update it, then process the rest
- which is a completely fine thing to do.

So for now - yeah, the correct thing to do is to just ignore
bv_offset/bv_len and go by PAGE_CACHE_SIZE. But - after immutable
biovecs is in, _then_ you'll be able to depend on bv_offset/bv_len
remaining unchanged (and you can get rid of your dependency on
PAGE_CACHE_SIZE bvecs).