[dm-devel] Re: [PATCH][RFC] Use bio markers for request callback
Jens Axboe
jens.axboe at oracle.com
Tue Sep 11 11:11:34 UTC 2007
On Tue, Sep 11 2007, Hannes Reinecke wrote:
> Hi all,
>
> this is a proposal for a different implementation of request callbacks. The
> existing ->endio callback of a request is actually a destructor function,
> to be called to terminate a request and free all structures.
>
> However, on certain occasions (like request-based multipathing) it is
> desirable to have a callback function for a request which is called right
> after the request is finished, ie in end_that_request_first() before any
> bio->bi_endio callback is called.
>
> So a simple solution for this is to clone the request and add a new
> 'marker' bio in front of the bio list of the request. This callback will be
> attached a structure in bi_private which keeps a pointer to the cloned and
> the original request, thus serving as a callback for the request itself.
>
> Proposed patch attached. As usual comments are welcome.
Honestly, I think this approach is a much worse design than the NEC
callback option. Exporting __make_request() (which is an INTERNAL
function) aside, this looks like one big hack with pseudo bio attached
to front of list, the enable/disable elevator stuff (does that even work
on stacked devices?).
--
Jens Axboe
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