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From:   David Laight <David.Laight@ACULAB.COM>
To:     'David Howells' <dhowells@redhat.com>
CC:     Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jens Axboe <axboe@kernel.dk>, "Christoph Hellwig" <hch@lst.de>,
        Christian Brauner <christian@brauner.io>,
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        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
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        John Hubbard <jhubbard@nvidia.com>
Subject: RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for
 UBUF/IOVEC
Thread-Topic: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for
 UBUF/IOVEC
Thread-Index: AQHZ51kQ26Nrcgq1TkqWWiI5nPv6dLAbdkPggAAj+YCAABqPYP//+bcAgAAdLuA=
Date:   Fri, 15 Sep 2023 12:10:07 +0000
Message-ID: <72e93605b28742c2a496ce4890ecaa80@AcuMS.aculab.com>
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From: David Howells
> Sent: 15 September 2023 12:23
> 
> David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > > > Some measurements can be made using readv() and writev()
> > > > on /dev/zero and /dev/null.
> > >
> > > Forget /dev/null; that doesn't actually engage any iteration code.  The same
> > > for writing to /dev/zero.  Reading from /dev/zero does its own iteration thing
> > > rather than using iterate_and_advance(), presumably because it checks for
> > > signals and resched.
> >
> > Using /dev/null does exercise the 'copy iov from user' code.
> 
> Ummm....  Not really:

I was thinking of import_iovec() - or whatever its current
name is.

That really needs a single structure that contains the iov_iter
and the cache[] (which the caller pretty much always allocates
in the same place).
Fiddling with that is ok until you find what io_uring does.
Then it all gets entirely horrid.

	David

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