Re: [RFC 0/8] Replacing the readpages a_op

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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.



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