Re: Extending page pinning into fs/direct-io.c

[Christoph Hellwig]

On Tue, May 23, 2023 at 09:16:11PM +0100, David Howells wrote:
> I've been poking at it this afternoon, but it doesn't look like it's going to
> be straightforward, unfortunately.  The mm folks have been withdrawing access
> to the pinning API behind the ramparts of the mm/ dir.  Further, the dio code
> will (I think), under some circumstances, arbitrarily insert the zero_page
> into a list of things that are maybe pinned or maybe unpinned, but I can (I
> think) also be given a pinned zero_page from the GUP code if the page tables
> point to one and a DIO-write is requested - so just doing if page == zero_page
> isn't sufficient.

Yes.  I think the proper workaround is to add a MM helper that just
pins a single page and make it available to direct-io.c.  It should not
be exported and clearly marked to not be used in new code.  

> What I'd like to do is to make the GUP code not take a ref on the zero_page
> if, say, FOLL_DONT_PIN_ZEROPAGE is passed in, and then make the bio cleanup
> code always ignore the zero_page.

I don't think that'll work, as we can't mix different pin vs get types
in a bio.  And that's really a good thing.

> Something that I noticed is that the dio code seems to wangle to page bits on
> the target pages for a DIO-read, which seems odd, but I'm not sure I fully
> understand the code yet.

I don't understand this sentence.