linux-kernel - Re: [git pull] iov

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Date:   Fri, 10 Sep 2021 09:08:02 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] iov_iter fixes

On 9/10/21 8:42 AM, Al Viro wrote:
> On Fri, Sep 10, 2021 at 07:57:49AM -0600, Jens Axboe wrote:
> 
>> It was just a quick hack, might very well be too eager to go through
>> those motions. But pondering this instead of sleeping, we don't need to
>> copy all of iov_iter in order to restore the state, and we can use the
>> same advance after restoring. So something like this may be more
>> palatable. Caveat - again untested, and I haven't tested the performance
>> impact of this at all.
> 
> You actually can cut it down even more - nr_segs + iov remains constant
> all along, so you could get away with just 3 words here...  I would be

Mmm, the iov pointer remains constant? Maybe I'm missing your point, but
the various advance functions are quite happy to increment iter->iov or
iter->bvec, so we need to restore them. From a quick look, looks like
iter->nr_segs is modified for advancing too.

What am I missing?

> surprised if extra memory traffic had shown up - it's well within the
> noise from register spills, (un)inlining, etc.  We are talking about
> 3 (or 4, with your variant) extra words on one stack frame (and that'd
> be further offset by removal of ->truncated); I'd still like to see the
> profiling data, but concerns about extra memory traffic due to that
> are, IMO, misplaced.

See other email that was just sent out, it is measurable but pretty
minimal. But that's also down to about 1/3rd of copying the whole
thing blindly, so definitely a better case.

-- 
Jens Axboe