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Date:   Sun, 22 May 2022 12:48:45 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        linux-fsdevel@vger.kernel.org
Subject: Re: [RFC] what to do with IOCB_DSYNC?
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On Sun, May 22, 2022 at 06:39:39AM -0600, Jens Axboe wrote:
> On 5/22/22 5:45 AM, Christoph Hellwig wrote:
> > On Sun, May 22, 2022 at 12:15:59PM +0100, Matthew Wilcox wrote:
> >>> 	Direct kernel pointer, surely?  And from a quick look,
> >>> iov_iter_is_kaddr() checks for the wrong value...
> >>
> >> Indeed.  I didn't test it; it was a quick patch to see if the idea was
> >> worth pursuing.  Neither you nor Christoph thought so at the time, so
> >> I dropped it.  if there are performance improvements to be had from
> >> doing something like that, it's a more compelling idea than just "Hey,
> >> this removes a few lines of code and a bit of stack space from every
> >> caller".
> > 
> > Oh, right I actually misremembered what the series did.  But something
> > similar except for user pointers might help with the performance issues
> > that Jens sees, and if it does it might be worth it to avoid having
> > both the legacy read/write path and the iter path in various drivers.
> 
> Right, ITER_UADDR or something would useful. I'll try and test that,
> should be easy to wire up.

Careful - it's not just iov_iter_advance() and __iterate_and_advance() (that
one should use the same "callback" argument as iovec case).  /dev/random is
not the only thing we use read(2) and write(2) on...

I can cook a patch doing that, just let me get some caffeine into the
bloodstream first...