linux-kernel - Re: [PATCH v4 0/3] random: convert to using iters, for Al Viro

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Date:   Fri, 20 May 2022 10:06:06 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Theodore Ts'o <tytso@....edu>, Christoph Hellwig <hch@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 0/3] random: convert to using iters, for Al Viro

On 5/20/22 10:03 AM, Jason A. Donenfeld wrote:
>> Then I'd say there are only two options:
>>
>> - Add a helper that provides splice for something that only has
>>   read/write set.
> 
> That'd be fine with me, but wouldn't it involve bringing back set_fs(),
> because of the copy_to_user() in there?

I haven't even looked into whether it's currently feasible or not, just
mentioning it as a potential option. But the better one is definetely
the next one...

>> - Just accept that we're 3% slower reading from /dev/urandom for now,
>>   and maybe 1-2% for small reads. Can't really imagine this being a huge
>>   issue, how many high throughput /dev/urandom read situations exist in
>>   the real world?
> 
> An option three might be that eventually the VFS overhead is worked out
> and read_iter() reaches parity. One can hope, I guess.

And that will certainly happen, especially as we have other paths that
don't really have the choice, they have to use the iterator versions.
And if we can get a bit closer, then that also opens the door more
generic conversions so we can kill ->read/->write for almost all cases
(except those weirdo ones that Al pointed out).

-- 
Jens Axboe