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