Re: [PATCH v7 2/8] iov_iter: Add a function to extract a page list from an iterator

[David Hildenbrand]

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