Re: New TRIM/UNMAP tree published (2009-05-02)

[Jeff Garzik <jeff@garzik.org>]

James Bottomley wrote:
> On Sun, 2009-05-03 at 15:20 -0400, Jeff Garzik wrote:
>> [tangent...]
>>
>> Does make you wonder if a ->init_rq_fn() would be helpful, one that 
>> could perform gfp_t allocations rather than GFP_ATOMIC?  The idea being 
>> to call ->init_rq_fn() almost immediately after creation of struct 
>> request, by the struct request creator.
> 
> Isn't that what the current prep_fn actually is?

> It's hard to see how ... prep_rq_fn is already called pretty early ...
> almost as soon as the elevator has decided to spit out the request

prep_rq_fn is called very, very late -- from elv_next_request(), which 
is called from ->request_fn.

As quoted above, I'm talking about something closer to -creation- time, 
not -execution- time.


>> The creator of struct request generally has more freedom to sleep, and 
>> it seems logical to give low-level drivers a "fill in LLD-specific info" 
>> hook BEFORE the request is ever added to a request_queue.
> 
> Unfortunately it's not really possible to find a sleeping context in
> there:  The elevators have to operate from the current
> elv_next_request() context, which, in most drivers can either be user or
> interrupt.

Sure, because that's further down the pipeline at the request execution 
stage.  Think more like make_request time...

> The ideal for REQ_TYPE_DISCARD seems to be to force a page allocation
> tied to a bio when it's issued at the top.  That way everyone has enough
> memory when it comes down the stack (both extents and WRITE SAME sector
> will fit into a page ... although only just for WRITE SAME on 4k
> sectors).

Makes sense...

	Jeff