Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[Andrea Arcangeli <aarcange@redhat.com>]

On Mon, Mar 16, 2009 at 05:44:25PM -0700, Linus Torvalds wrote:
> -		reuse = reuse_swap_page(old_page);
> +		/*
> +		 * If we can re-use the swap page _and_ the end
> +		 * result has only one user (the mapping), then
> +		 * we reuse the whole page
> +		 */
> +		if (reuse_swap_page(old_page))
> +			reuse = page_count(old_page) == 1;
>  		unlock_page(old_page);

Think if the anon page is added to swapcache and the pte is unmapped
by the VM and set non present after GUP taken the page for a O_DIRECT
read (write to memory). If a thread writes to the page while the
O_DIRECT read is running in another thread (or aio), then do_wp_page
will make a copy of the swapcache under O_DIRECT read, and part of the
read operation will get lost.

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