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

["Darrick J. Wong" <darrick.wong@oracle.com>]

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:
> > On Fri, Mar 21, 2014 at 07:32:16PM -0700, Darrick J. Wong wrote:
> > > On Fri, Mar 21, 2014 at 05:29:09PM -0700, Zach Brown wrote:
> > > > 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 <jack@suse.cz>
> SUSE Labs, CR
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