Re: bio_chain: proposed solution for bio_alloc failure and large IO simplification

[Adam J. Richter]

On Fri, 14 Jun 2002 Jens Axboe wrote:
>On Thu, Jun 13 2002, Adam J. Richter wrote:
[...]
>> 	newbio = bio_chain(oldbio);
[...]
>> 	I realize there may be locking issues in implementing this.

>but I think that statement is very naive :-)

>Essentially you are keeping a cookie to the old bio submitted, so that
>you can merge new bio with it fairly cheaply (it's still not very cheap,
>although you do eliminate the biggest offender, the list merge scan).
>The first problem is that once you submit the bio, there are some things
>that are not yours to touch anymore. You cannot change bi_next for
>instance, you have no idea what state the bio is in wrt I/O completion.
>Sure you can hold a reference to the bio, but all that really gets you
>in this situation is a handle to it, nothing more. How do you decide
>when to invalidate this cookie that you have?

	At any time, there could be only one "hinted" bio in a
request: the last bio in the request.  So you only have to
clear the hint when:

		1. you merge bio's,
		2. elv_next_request is called,
		3. newbio is submitted.

	In all three cases q->queue_lock gets taken, so we should
not need to add any additional spin_lock_irq's, and the two lines
to clear the hint pointers should be trivial.

>So what do I like? As I see it, the only nice way to make sure that a
>bio always has the best possible size is to just ask 'can we put another
>page into this bio'. That way we can apply the right (and not just
>minimum) limits every time.

	That would be pretty nice too, but that lacks three potential
advantages of my bio_chain appoach, in order of importance:

		1. bio_chain is avoids memory allocation failures,
		2. bio_chain can start the first IO slightly sooner,
		3. bio_chain makes bio submitters slightly simpler.

	The disadvantage of my single I/O vector bio_chain apprach is
that it results in another procedure call per vector.  We also already
have code that builds multiple IO vectors in ll_rw_kio and the mpage.c
routines.

	I think I would be happy enough with your approach, but, just
to eliminate possibilities of memory allocation failure, I think I
want to still have some kind of bio_chain, perhaps without the merge
hinting, but with a parameter to allow for allocating a bio up to the
size of oldbio, like so:

struct bio *bio_chain(struct bio *oldbio, int gfp_mask,
		       int nvecs /* <= oldbio->bi_max */);

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