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 14:11:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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

On 23.01.23 12:51, David Howells wrote:
> David Hildenbrand <david@...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>

FOLL_PIN users are fine in that regard, because we properly detect 
"maybe pinned" during fork() and copy the page. See 
tools/testing/selftests/mm/cow.c (still called 
tools/testing/selftests/vm/cow.c upstream IIRC) for some test cases for 
that handling.

FOLL_GET does not work as expected in that regard: pages can't be 
detected as pinned and we won't be copying them during fork(). We'll end 
up COW-sharing them, which can result in trouble later.

Switching from FOLL_GET to FOLL_PIN was in the works by John H. Not sure 
what the status is. Interestingly, 
Documentation/core-api/pin_user_pages.rst already documents that "CASE 
1: Direct IO (DIO)" uses FOLL_PIN ... which does, unfortunately, no 
reflect reality yet.

-- 
Thanks,

David / dhildenb