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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <3814749.1674474663@warthog.procyon.org.uk>
References: <3814749.1674474663@warthog.procyon.org.uk> <246ba813-698b-8696-7f4d-400034a3380b@redhat.com> <20230120175556.3556978-1-dhowells@redhat.com> <20230120175556.3556978-3-dhowells@redhat.com>
Cc:     dhowells@redhat.com, David Hildenbrand <david@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        John Hubbard <jhubbard@nvidia.com>, linux-mm@kvack.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator
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Date:   Mon, 23 Jan 2023 12:00:46 +0000
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David Howells <dhowells@redhat.com> wrote:

> > How does this work align with the goal of no longer using FOLL_GET for
> > O_DIRECT? We should get rid of any FOLL_GET usage for accessing page c=
ontent.
> =

> Would that run the risk of changes being made by the child being visible=
 to
> the a DIO write if the parent changes the buffer first?
> =

> =

> 	PARENT			CHILD
> 	=3D=3D=3D=3D=3D=3D			=3D=3D=3D=3D=3D
> 	start-DIO-write
> 	fork() =3D pid		fork() =3D 0
> 	alter-buffer
> 	CoW happens
> 	page copied		original page retained
> 				alter-buffer
> 		<DMA-happens>

Ah, I think I might have got the wrong end of the stick.  A pinned page is
*always* copied on fork() if I understand copy_present_pte() correctly.

David