Re: [RFC] Changing COW detection to be memory hotplug friendly

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Feb 11, 2005 at 07:23:09AM +0000, Hugh Dickins wrote:
> And it's fine for the behaviour to be somewhat undefined in this
> peculiar case: the important thing is just that the page must not
> be freed and reused while I/O occurs, hence get_user_page raising
> the page_count - which I'm _not_ proposing to change!

Ok, I'm quite convinced it's correct now. The only thing that can make
mapcount go up without the lock on the page without userspace
intervention (and userspace intervention would make it an undefined
behaviour like in my example with fork), was the swapin, and you covered
it by moving the unlock after page_add_anon_rmap (so mapcount changes
atomically with the page_swapcount there too). Swapoff was already doing
it under the page lock.

Then we should use the mapcount/swapcount in remove_exclusive_swap_page
too.