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
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