On Mon, Jan 13, 2020 at 10:34:16PM +0000, Chris Mason wrote: > On 13 Jan 2020, at 17:27, 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: > >>>>> 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. > >> > >> 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. > > Both btrfs and xfs do this, we have a bio that we pass around and build > and submit. We both also do some gymnastics in writepages to avoid > waiting for the bios we've been building to finish while we're building > them. > > I love the idea of the plug api having a way to hold that for us, but > sometimes we really are building the bios, and we don't want the plug to > let it go if we happen to schedule. The plug wouldn't have to let the bio go. I appreciate the plug does let requests go on context switch, but it wouldn't have to let the bio go. This bio is being stored on the stack, just as now, so it's still there.
