Re: [11/36] Use page_cache_xxx in fs/buffer.c

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Aug 31 2007, Christoph Lameter wrote:
> On Fri, 31 Aug 2007, Jens Axboe wrote:
> 
> > They have nothing to do with each other, you are mixing things up. It
> > has nothing to do with the device being able to dma into that memory or
> > not, we have fine existing infrastructure to handle that. But different
> > hardware have different characteristics on what a single segment is. You
> > can say "a single segment cannot cross a 32kb boundary". So from the
> > example above, your single 64k page may need to be split into two
> > segments. Or it could have a maximum segment size of 32k, in which case
> > it would have to be split as well.
> > 
> > Do you see what I mean now?
> 
> Ok. So another solution maybe to limit the blocksizes that can be used 
> with a device?

That'd work for creation, but not for moving things around.

> > > How do we split that up today? We could add processing to submit_bio
> > > to check for the boundary and create two bios.
> > 
> > But we do not split them up today - see what I wrote! Today we impose
> > the restriction that a device must be able to handle a single "normal"
> > page, and if it can't do that, it has to split it up itself.
> > 
> > But yes, you would have to create some out-of-line function to use
> > bio_split() until you have chopped things down enough. It's not a good
> > thing for performance naturally, but if we consider this a "just make it
> > work" fallback, I don't think it's too bad. You want to make a note of
> > that it is happening though, so people realize that it is happening.
> 
> Hmmmm.. We could keep the existing scheme too and check that device 
> drivers split things up if they are too large? Isnt it possible today
> to create a huge bio of 2M for huge pages and send it to a device?

Not sure, aren't the constituents of compound pages the basis for IO?

-- 
Jens Axboe