Re: [RFC PATCH 0/5] userspace PI passthrough via AIO/DIO

On Sun, Mar 23, 2014 at 03:02:44PM +0100, Jan Kara wrote:

On Sat 22-03-14 02:43:20, Darrick J. Wong wrote:

quoted

On Fri, Mar 21, 2014 at 07:32:16PM -0700, Darrick J. Wong wrote:

quoted

On Fri, Mar 21, 2014 at 05:29:09PM -0700, Zach Brown wrote:

quoted

I'll admit, though, that I don't really like having to fetch the 'has'
bits first to find out how large the rest of the struct is.  Maybe
that's not worth worrying about.

I'm not worrying about having to pluck 'has' out of the structure, but needing
a function to tell me how big of a buffer I need for a given pile of flags
seems ... icky.  But maybe the ease of modifying strace and security auditors
would make it worth it?

How about explicitly specifying the structure size in struct some_more_args,
and checking that against whatever we find in .has?  Hm.  I still think that's
too clever for my brain to keep together for long.

I'm also nervous that we could be creating this monster of a structure wherein
some user wants to tack the first and last hints ever created onto an IO, so
now we have to lug this huge structure around that has space for hints that
we're not going to use, and most of which is zeroes.

  Well, why does it matter that the structure would be big? Are do you
think the memory consumption would matter?

I doubt the memory consumption will be a big deal (compared to the size of the
IOs), but I'm a little concerned about the overhead of copying a mostly-zeroes
user buffer into the kernel.  I guess it's not a big deal to copy the whole
thing now and if people complain about the overhead, switch it to let the IO
attribute controllers selectively copy_from_user later.

--D

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR
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