On Mon, 13 May 2013 11:21:21 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:
> If a page is on a pagevec then it is !PageLRU and mark_page_accessed()
> may fail to move a page to the active list as expected. Now that the LRU
> is selected at LRU drain time, mark pages PageActive if they are on the
> local pagevec so it gets moved to the correct list at LRU drain time.
> Using a debugging patch it was found that for a simple git checkout based
> workload that pages were never added to the active file list in practice
> but with this patch applied they are.
>
> before after
> LRU Add Active File 0 750583
> LRU Add Active Anon 2640587 2702818
> LRU Add Inactive File 8833662 8068353
> LRU Add Inactive Anon 207 200
>
> Note that only pages on the local pagevec are considered on purpose. A
> !PageLRU page could be in the process of being released, reclaimed, migrated
> or on a remote pagevec that is currently being drained. Marking it PageActive
> is vunerable to races where PageLRU and Active bits are checked at the
> wrong time. Page reclaim will trigger VM_BUG_ONs but depending on when the
> race hits, it could also free a PageActive page to the page allocator and
> trigger a bad_page warning. Similarly a potential race exists between a
> per-cpu drain on a pagevec list and an activation on a remote CPU.
>
> lru_add_drain_cpu
> __pagevec_lru_add
> lru = page_lru(page);
> mark_page_accessed
> if (PageLRU(page))
> activate_page
> else
> SetPageActive
> SetPageLRU(page);
> add_page_to_lru_list(page, lruvec, lru);
>
> In this case a PageActive page is added to the inactivate list and later the
> inactive/active stats will get skewed. While the PageActive checks in vmscan
> could be removed and potentially dealt with, a skew in the statistics would
> be very difficult to detect. Hence this patch deals just with the common case
> where a page being marked accessed has just been added to the local pagevec.
but but but
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -431,6 +431,27 @@ void activate_page(struct page *page)
> }
> #endif
>
> +static void __lru_cache_activate_page(struct page *page)
> +{
> + struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
> + int i;
> +
> + /*
> + * Search backwards on the optimistic assumption that the page being
> + * activated has just been added to this pagevec
> + */
> + for (i = pagevec_count(pvec) - 1; i >= 0; i--) {
> + struct page *pagevec_page = pvec->pages[i];
> +
> + if (pagevec_page == page) {
> + SetPageActive(page);
> + break;
> + }
> + }
> +
> + put_cpu_var(lru_add_pvec);
> +}
> +
> /*
> * Mark a page as having seen activity.
> *
> @@ -441,8 +462,17 @@ void activate_page(struct page *page)
> void mark_page_accessed(struct page *page)
> {
> if (!PageActive(page) && !PageUnevictable(page) &&
> - PageReferenced(page) && PageLRU(page)) {
> - activate_page(page);
> + PageReferenced(page)) {
> +
> + /*
> + * If the page is on the LRU, promote immediately. Otherwise,
> + * assume the page is on a pagevec, mark it active and it'll
> + * be moved to the active LRU on the next drain
> + */
> + if (PageLRU(page))
> + activate_page(page);
> + else
> + __lru_cache_activate_page(page);
> ClearPageReferenced(page);
> } else if (!PageReferenced(page)) {
> SetPageReferenced(page);
For starters, activate_page() doesn't "promote immediately". It sticks
the page into yet another pagevec for deferred activation.
Also, I really worry about the fact that
activate_page()->drain->__activate_page() will simply skip over the
page if it has PageActive set! So PageActive does something useful if
the page is in the add-to-lru pagevec but nothing useful if the page is
in the activate-it-soon pagevec. This is a confusing, unobvious bug
attractant.
Secondly, I really don't see how this code avoids the races. Suppose
the page gets spilled from the to-add-to-lru pagevec and onto the real
LRU while mark_page_accessed() is concurrently executing. We end up
setting PageActive on a page which is on the inactive LRU? Maybe this
is a can't-happen, in which case it's nowhere near clear enough *why*
this can't happen.
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