Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Mon, Jan 16, 2023 at 11:08:24PM +0000, David Howells wrote:
> > Define FOLL_SOURCE_BUF and FOLL_DEST_BUF to indicate to get_user_pages*()
> > and iov_iter_get_pages*() how the buffer is intended to be used in an I/O
> > operation. Don't use READ and WRITE as a read I/O writes to memory and
> > vice versa - which causes confusion.
> >
> > The direction is checked against the iterator's data_source.
>
> Why can't we use the existing FOLL_WRITE?
Because FOLL_WRITE doesn't mean the same as WRITE:
(1) It looks like it should really be FOLL_CHECK_PTES_WRITABLE. It's not
defined as being anything to do with the I/O.
(2) The reason Al added ITER_SOURCE and ITER_DEST is that the use of READ and
WRITE with the iterators is confusing and kind of inverted - and the same
would apply with using FOLL_WRITE:
if (rw == READ)
gup_flags |= FOLL_WRITE;
So my thought is to make how you are using the buffer described by the
iterator explicit: "I'm using it as a source buffer" or "I'm using it as a
destination buffer".
Also, I don't want it to be FOLL_WRITE or 0. I want it to be written
explicitly in both cases. If you're going to insist on using FOLL_WRITE, then
there should be a FOLL_READ to go with it, even if it's #defined to 0.
David
