Re: [PATCH 2/7] Define new syscalls readv2,preadv2,writev2,pwritev2

[Zach Brown <zab@zabbo.net>]

On Wed, Sep 17, 2014 at 12:59:30PM -0400, Theodore Ts'o wrote:
> On Wed, Sep 17, 2014 at 12:05:23PM -0400, Milosz Tanski wrote:
> > Theodore,
> > 
> > I might be missing understanding something, but... I already omitted
> > read2 and write2 which can be implemented in userspace by libc (as you
> > pointed out). In the case of readv vs. preadv there's an extra
> > positional argument (file offset) and preadv version doesn't change
> > the file location. I didn't want to overload the meaning of preadv2 to
> > take a special negative offset value that uses the current file
> > position but also changes the file position.
> 
> off_t has to be signed, so having a magic negative value doesn't
> bother me that much.  Or you could use a flag bitvalue which means to
> use the fd's offset and to ignore the positional value.  (More
> bike-shedding :-)

splice has already set the precedent for an optionally specified offset
that falls back to the files's position.

static long do_splice(struct file *in, loff_t __user *off_in,
                      struct file *out, loff_t __user *off_out,
                      size_t len, unsigned int flags)
{
...
                if (off_out) {
                        if (copy_from_user(&offset, off_out, sizeof(loff_t)))
                                return -EFAULT;
                } else {
                        offset = out->f_pos;
                }

...
                if (!off_out)
                        out->f_pos = offset;
                else if (copy_to_user(off_out, &offset, sizeof(loff_t)))
                        ret = -EFAULT;

It's nice and simple and lets you update the user's offset.

> So the suggestion was one of trying to (probably fruitlessly) trying
> to stem the expnoential increase in read/write system calls.  :-)

I support this windmill tilting :). 

- z