| From | David Howells <> | Subject | Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into iov_iter_get_pages*() | Date | Tue, 17 Jan 2023 08:44:16 +0000 |
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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
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