Re: [PATCH 3/5] fs: remove ki_nbytes

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Feb 05, 2015 at 09:24:32AM +0100, Robert Baldyga wrote:

> No, function/f_fs.c and legacy/inode.c are in class (1). They have
> datagram semantics - each vector element is submitted in separate USB
> request. Each single request is sent in separate USB data packet (for
> bulk endpoints it can be more than one packet). In fact sync
> read()/write() also will give different results while called once with
> some block of data or in loop with the same block of data splitted into
> a few parts.

No, they don't.  This is from ffs_epfile_io():

                data = kmalloc(data_len, GFP_KERNEL);
                if (unlikely(!data))
                        return -ENOMEM;
                if (io_data->aio && !io_data->read) {
                        int i;
                        size_t pos = 0;
                        for (i = 0; i < io_data->nr_segs; i++) {
                                if (unlikely(copy_from_user(&data[pos],
                                             io_data->iovec[i].iov_base,
                                             io_data->iovec[i].iov_len))) {
                                        ret = -EFAULT;
                                        goto error;
                                }
                                pos += io_data->iovec[i].iov_len;
                        }

and that's the last point where it looks at iovec.  After that all work
is done to the copy in data, where no information about the boundaries
survives.  And ep_aio_write() (in legacy/inode.c) is the same way.

	You are confusing datagram-per-syscall (which they are) with
datagram-per-iovec (which they are definitely not).  IOW, they behave
as UDP sockets - writev() is purely scatter-gather variant of write(),
with datagram per syscall and all vector elements silently concatenated.
That's class 2, and _not_ in its intersection with class 1.

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