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

[David Howells]

David Howells <dhowells@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>

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