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Date:   Tue, 23 May 2023 22:55:28 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Jens Axboe <axboe@kernel.dk>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Hillf Danton <hdanton@sina.com>,
        Christian Brauner <brauner@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: Extending page pinning into fs/direct-io.c
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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.