Re: [PATCH 04/21] mm: zap_pte_range dont dirty anon

[Andrew Morton <akpm@osdl.org>]

Hugh Dickins <hugh@veritas.com> wrote:
>
> > What is the page is (for example) clean swapcache, having been recently
>  > faulted in.  If this pte indicates that this process has modified the page
>  > and we don't run set_page_dirty(), the page could be reclaimed and the
>  > change is lost.
> 
>  Absolutely.  But either the page is unique to this mm, shared only with
>  swapcache: in which case we're about to do a free_swap_cache on it (that
>  may be delayed in actually freeing the swap because of not getting page
>  lock, presumably because vmscan just got to it, but no matter), and we
>  don't care at all that the page no longer represents what's on swap disk.
> 
>  Or, the page is shared with another mm.  But it's an anonymous page
>  (a private page), so it's shared via fork, and COW applies to it.

	mmap(MAP_ANONYMOUS|MAP_SHARED)
	fork()
	swapout
	swapin
	swapoff

Now we have two mm's sharing a clean, non-cowable, non-swapcache anonymous
page, no?
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