On Mon, Jan 13, 2020 at 03:14:26PM -0700, Jens Axboe wrote: > On 1/13/20 3:10 PM, Matthew Wilcox wrote: > > On Mon, Jan 13, 2020 at 03:00:40PM -0700, Jens Axboe wrote: > >> On 1/13/20 2:58 PM, Matthew Wilcox wrote: > >>> On Mon, Jan 13, 2020 at 06:00:52PM +0000, Chris Mason wrote: > >>>> This is true, I didn't explain that part well ;) Depending on > >>>> compression etc we might end up poking the xarray inside the actual IO > >>>> functions, but the main difference is that btrfs is building a single > >>>> bio. You're moving the plug so you'll merge into single bio, but I'd > >>>> rather build 2MB bios than merge them. > >>> > >>> Why don't we store a bio pointer inside the plug? You're opencoding that, > >>> iomap is opencoding that, and I bet there's a dozen other places where > >>> we pass a bio around. Then blk_finish_plug can submit the bio. > >> > >> Plugs aren't necessarily a bio, they can be callbacks too. > > > > I'm thinking something as simple as this: > > It's a little odd imho, the plugging generally collect requests. Sounds > what you're looking for is some plug owner private data, which just > happens to be a bio in this case? > > Is this over repeated calls to some IO generating helper? Would it be > more efficient if that helper could generate the full bio in one go, > instead of piecemeal? The issue is around ->readpages. Take a look at how iomap_readpages works, for example. We're under a plug (taken in mm/readahead.c), but we still go through the rigamarole of keeping a pointer to the bio in ctx.bio and passing ctx around so that we don't end up with many fragments which have to be recombined into a single bio at the end. I think what I want is a bio I can reach from current, somehow. And the plug feels like a natural place to keep it because it's basically saying "I want to do lots of little IOs and have them combined". The fact that the iomap code has a bio that it precombines fragments into suggests to me that the existing antifragmentation code in the plugging mechanism isn't good enough. So let's make it better by storing a bio in the plug and then we can get rid of the bio in the iomap code.
