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

[Al Viro]

On Thu, Jan 19, 2023 at 12:15:44AM +0000, Al Viro wrote:
> 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...

PS: Documentation/driver-api/pci/p2pdma.rst seems to imply that those
pages should not be possible to mmap, so either that needs to be
updated, or... how the hell could we run into those in g-u-p,
anyway?  Really confused...