linux-kernel - 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
From:   David Howells <dhowells@...hat.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     dhowells@...hat.com, David Hildenbrand <david@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>, linux-mm@...ck.org
Subject: Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

David Howells <dhowells@...hat.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 content.
> 
> 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
> 	======			=====
> 	start-DIO-write
> 	fork() = pid		fork() = 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