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Date:   Sat, 25 Mar 2023 04:46:54 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-fsdevel@vger.kernel.org, torvalds@linux-foundation.org,
        brauner@kernel.org
Subject: Re: [PATCHSET 0/2] Turn single segment imports into ITER_UBUF
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On Fri, Mar 24, 2023 at 02:44:41PM -0600, Jens Axboe wrote:
> Hi,
> 
> We've been doing a few conversions of ITER_IOVEC to ITER_UBUF in select
> spots, as the latter is cheaper to iterate and hence saves some cycles.
> I recently experimented [1] with io_uring converting single segment READV
> and WRITEV into non-vectored variants, as we can save some cycles through
> that as well.
> 
> But there's really no reason why we can't just do this further down,
> enabling it for everyone. It's quite common to use vectored reads or
> writes even with a single segment, unfortunately, even for cases where
> there's no specific reason to do so. From a bit of non-scientific
> testing on a vm on my laptop, I see about 60% of the import_iovec()
> calls being for a single segment.
> 
> I initially was worried that we'd have callers assuming an ITER_IOVEC
> iter after a call import_iovec() or import_single_range(), but an audit
> of the kernel code actually looks sane in that regard. Of the ones that
> do call it, I ran the ltp test cases and they all still pass.

Which tree was that audit on?  Mainline?  Some branch in block.git?