On 1/13/20 3:27 PM, Matthew Wilcox wrote: > 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. But it doesn't fit the plug model very well. If you get preempted, it could go away. Or if you have nested plugs, it also won't work at all. I think it's much saner to keep the "current" bio _outside_ of the plug, and work on it in peace until you want to submit it. -- Jens Axboe
