Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into iov_iter_get_pages*()

[Al Viro]

On Wed, Jan 18, 2023 at 11:03:52PM +0000, Al Viro wrote:
> On Mon, Jan 16, 2023 at 11:57:08PM -0800, Christoph Hellwig 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?
> 
> 	I'm really not fond of passing FOLL_... stuff into iov_iter
> primitives.  That space contains things like FOLL_PIN, which makes
> no sense whatsoever for non-user-backed iterators; having the
> callers pass it in makes them automatically dependent upon the
> iov_iter flavour.

Actually, looking at that thing...  Currently we use it only for
FOLL_PCI_P2PDMA.  It alters behaviour of get_user_pages_fast(), but...
it is completely ignored for ITER_BVEC or ITER_PIPE.  So how the
hell is it supposed to work?

And ITER_BVEC *can* get there.  blkdev_direct_IO() can get anything
->write_iter() can get, and io_uring will feed stuff to it.  For
that matter, ->read_iter() can lead to it as well, so
generic_file_splice_read() can end up passing ITER_PIPE to that
sucker.

Could somebody give a braindump on that thing?  It looks like we
have pages that should not be DMA'd to/from unless driver takes
some precautions and we want to make sure they won't be fed to
drivers that don't take such.  With checks done in a very odd
place...