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

From: Jens Axboe (
Date: Sat Jun 15 2002 - 04:14:05 EST

On Sat, Jun 15 2002, Adam J. Richter wrote:
> >> So, I need a fourth location at in generic_make_request
> >> just before the call to q->make_request_fn, like so:
> >>
> >> if (q->make_request_fn != __make_request) {
> >> int flags;
> >> spin_lock_irqsave(q->lock, flags);
> >> clear_hint(bio);
> >> spin_unlock_irqrestore(q->lock, flags);
> >> }
> >> ret = q->make_request_fn(q, bio);
> >Irk, this is ugly. But how you are moving away from the initial goal (or
> >maybe this was your goal the whole time, just a single merge hint?) of
> >passing back the hint instead of maintaing it in the queue. So let me
> >ask, are you aware of the last_merge I/O scheduler hint? Which does
> >exactly this already...
> The code that I think I more or less have in my head has not
> changed (aside from that fourth test).
> Although I was not aware of q->last_merge, I see that it is
> not what I want. I want up to one hint per request, for the last bio
> in the request. bi_hint would be null for all bio's except possibly
> the last bio in a request. I do not want just one hint per queue.

.. which, as far as I can see, brings us right back into the problems
with stalling the i/o and other nastiness. So far your approach seems
hackish at best, but ->

> If it is unclear what I mean, perhaps I really need to code it
> up to explain it. and we can discuss it from there.

go ahead and code it up if you want and we can discuss it some more.

Jens Axboe

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