lkml.org 
[lkml]   [2023]   [Jan]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v6 03/34] iov_iter: Pass I/O direction into iov_iter_get_pages*()
Date
Christoph Hellwig <hch@infradead.org> 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

\
 
 \ /
  Last update: 2023-03-26 23:43    [W:0.365 / U:0.808 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site