Re: [PATCH] Check PageActive when evictable page and unevicetablepage race happen

[KOSAKI Motohiro]

2011/4/24 Minchan Kim <minchan.kim@xxxxxxxxx>:
> In putback_lru_page, unevictable page can be changed into evictable
> 's one while we move it among lru. So we have checked it again and
> rescued it. But we don't check PageActive, again. It could add
> active page into inactive list so we can see the BUG in isolate_lru_pages.
> (But I didn't see any report because I think it's very subtle)
>
> It could happen in race that zap_pte_range's mark_page_accessed and
> putback_lru_page. It's subtle but could be possible.
>
> Note:
> While I review the code, I found it. So it's not real report.
>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
> Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> ---
>  mm/vmscan.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b3a569f..c0cd1aa 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -562,7 +562,7 @@ int remove_mapping(struct address_space *mapping, struct page *page)
>  void putback_lru_page(struct page *page)
>  {
>        int lru;
> -       int active = !!TestClearPageActive(page);
> +       int active;
>        int was_unevictable = PageUnevictable(page);
>
>        VM_BUG_ON(PageLRU(page));
> @@ -571,6 +571,7 @@ redo:
>        ClearPageUnevictable(page);
>
>        if (page_evictable(page, NULL)) {
> +               active = !!TestClearPageActive(page);
>                /*
>                 * For evictable pages, we can use the cache.
>                 * In event of a race, worst case is we end up with an
> @@ -584,6 +585,7 @@ redo:
>                 * Put unevictable pages directly on zone's unevictable
>                 * list.
>                 */
> +               ClearPageActive(page);
>                lru = LRU_UNEVICTABLE;
>                add_page_to_unevictable_list(page);

I think we forgot 'goto redo' case. following patch is better?

------------------------------------------------
        if (page_evictable(page, NULL)) {
                /*
                 * For evictable pages, we can use the cache.
                 * In event of a race, worst case is we end up with an
                 * unevictable page on [in]active list.
                 * We know how to handle that.
                 */
                lru = active + page_lru_base_type(page);
+              if (active)
+                   SetPageActive(page);
                lru_cache_add_lru(page, lru);
--
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