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From: Rik van Riel on 22 Jul 2010 09:20
On 07/22/2010 03:41 AM, KAMEZAWA Hiroyuki wrote:
> Rik, how do you think ?
>
> ==
> From: KAMEZAWA Hiroyuki<kamezawa.hiroyu(a)jp.fujitsu.com>
>
> Problem: wrong BUG_ON() in __page_set_anon_rmap().
> Kernel version: mmotm-0719

> Description:
> Even if SwapCache is fully unmapped and mapcount goes down to 0,
> page->mapping is not cleared and will remain on memory until kswapd or some
> finds it. If a thread cause a page fault onto such "unmapped-but-not-discarded"
> swapcache, it will see a swap cache whose mapcount is 0 but page->mapping has a
> valid value.
>
> When it's reused at do_swap_page(), __page_set_anon_rmap() is called with
> "exclusive==1" and hits BUG_ON(). But this BUG_ON() is wrong. Nothing bad
> with rmapping a page which has page->mapping isn't 0.

Yes, you are absolutely right.

Acked-by: Rik van Riel <riel(a)redhat.com>

> Index: mmotm-2.6.35-0719/mm/rmap.c
> ===================================================================
> --- mmotm-2.6.35-0719.orig/mm/rmap.c
> +++ mmotm-2.6.35-0719/mm/rmap.c
> @@ -783,8 +783,16 @@ static void __page_set_anon_rmap(struct
> if (PageAnon(page))
> return;
> anon_vma = anon_vma->root;
> - } else
> - BUG_ON(PageAnon(page));
> + } else {
> + /*
> + * In this case, swapped-out-but-not-discarded swap-cache
> + * is remapped. So, no need to update page->mapping here.
> + * We convice anon_vma poitned by page->mapping is not obsolete
> + * because vma->anon_vma is necessary to be a family of it.
> + */
> + if (PageAnon(page))
> + return;
> + }
>
> anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
> page->mapping = (struct address_space *) anon_vma;
>


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