Hello,

On Tue, Feb 01, 2011 at 12:38:46PM -0500, Mike Snitzer wrote:

quoted

I thought about doing it this way but I think we're burying the
REQ_FLUSH|REQ_FUA test logic too deep.  get_request() shouldn't
"magically" know not to allocate elevator data.

There is already a considerable amount of REQ_FLUSH|REQ_FUA special
casing magic sprinkled though-out the block layer.  Why is this
get_request() change the case that goes too far?

After the reimplementation, FLUSH implementation seems to be pretty
well isolated.  Also, having REQ_FLUSH logic in the issue and
completion paths is logical and preventing them from leaking to other
places sounds like a good idea.

quoted

The decision should
be made higher in the stack and passed down to get_request().  e.g. if
REQ_SORTED is set in @rw, elevator data is allocated; otherwise, not.

Considering REQ_SORTED is set in elv_insert(), well after get_request() 
is called, I'm not seeing what you're suggesting.

I was suggesting using REQ_SORTED in @rw parameter to indicate "this
request may be sorted and thus needs elevator data allocation".

Anyway, I agree that ideally we'd have a mechanism to explicitly
short-circuit elevator initialization.  But doing so in a meaningful way
would likely require a fair amount of refactoring of get_request* and
its callers.  I'll come back to this and have another look but my gut is
this interface churn wouldn't _really_ help -- all things considered.

I don't know.  I agree that it's not a critical issue but, to me,
subjectively of course, it feels a bit too subtle.  The sharing of
fields using unions is already subtle enough.  I with that at least
the allocation switching would be obvious and explicit.  The combined
subtleties scare me.

Thank you.

-- 
tejun