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 - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
