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

[Andrew Morton <akpm@osdl.org>]

Hugh Dickins <hugh@veritas.com> wrote:
>
> zap_pte_range already avoids wasting time to mark_page_accessed on anon
> pages: it can also skip anon set_page_dirty - the page only needs to be
> marked dirty if shared with another mm, but that will say pte_dirty too.
> 

Are you sure about this?

> --- mm03/mm/memory.c	2005-09-24 19:26:38.000000000 +0100
> +++ mm04/mm/memory.c	2005-09-24 19:27:05.000000000 +0100
> @@ -574,12 +574,14 @@ static void zap_pte_range(struct mmu_gat
>  						addr) != page->index)
>  				set_pte_at(tlb->mm, addr, pte,
>  					   pgoff_to_pte(page->index));
> -			if (pte_dirty(ptent))
> -				set_page_dirty(page);
>  			if (PageAnon(page))
>  				dec_mm_counter(tlb->mm, anon_rss);
> -			else if (pte_young(ptent))
> -				mark_page_accessed(page);
> +			else {
> +				if (pte_dirty(ptent))
> +					set_page_dirty(page);
> +				if (pte_young(ptent))
> +					mark_page_accessed(page);
> +			}
>  			tlb->freed++;
>  			page_remove_rmap(page);
>  			tlb_remove_page(tlb, page);

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.

Or what is the page was an anon page resulting from (say) a swapoff, and
it's shared by two mm's and one has modified it and we drop that dirty pte?

Or <other scenarios>.

Need more convincing.
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