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

On Mon, Jan 13, 2020 at 10:34:16PM +0000, Chris Mason wrote:

On 13 Jan 2020, at 17:27, Matthew Wilcox wrote:

quoted

On Mon, Jan 13, 2020 at 03:14:26PM -0700, Jens Axboe wrote:

quoted

On 1/13/20 3:10 PM, Matthew Wilcox wrote:

quoted

On Mon, Jan 13, 2020 at 03:00:40PM -0700, Jens Axboe wrote:

quoted

On 1/13/20 2:58 PM, Matthew Wilcox wrote:

quoted

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.