linux-kernel - Re: [git pull] iov

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Date:   Fri, 10 Sep 2021 18:48:34 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        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 Fri, Sep 10, 2021 at 10:31:00AM -0700, Linus Torvalds wrote:
> On Fri, Sep 10, 2021 at 10:26 AM Jens Axboe <axboe@...nel.dk> wrote:
> >
> > On 9/10/21 10:58 AM, Linus Torvalds wrote:
> > > On Fri, Sep 10, 2021 at 9:56 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> > >>
> > >> What's the point of all those contortions, anyway?  You only need it for
> > >> iovec case; don't mix doing that and turning it into flavour-independent
> > >> primitive.
> > >
> > > Good point, making it specific to iovec only gets rid of a lot of
> > > special cases and worries.
> > >
> > > This is fairly specialized, no need to always cater to every possible case.
> >
> > Alright, split into three patches:
> >
> > https://git.kernel.dk/cgit/linux-block/log/?h=iov_iter
> 
> That looks sane to me.
> 
> Please add some comment about how that
> 
>         i->iov -= state->nr_segs - i->nr_segs;
> 
> actually is the right thing for all the three cases (iow how 'iov',
> 'kvec' and 'bvec' all end up having a union member that acts the same
> way).
> 
> But yeah, I like how the io_uring.c code looks better this way too.
> 
> Al, what do you think?

I think that sizeof(struct bio_vec) != sizeof(struct iovec):
struct bio_vec {
        struct page     *bv_page;
	unsigned int    bv_len;
	unsigned int    bv_offset;
};
takes 3 words on 32bit boxen.
struct iovec
{
        void __user *iov_base;  /* BSD uses caddr_t (1003.1g requires void *) */
	__kernel_size_t iov_len; /* Must be size_t (1003.1g) */
};
takes 2 words on 32bit boxen.