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Subject[PATCH 10/32] readahead: rescue_pages()
Introduce function rescue_pages() to protect pages in danger of thrashing.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
mm/readahead.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 87 insertions(+)
--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -682,6 +682,93 @@ unsigned long max_sane_readahead(unsigne
}

/*
+ * Adaptive read-ahead.
+ *
+ * Good read patterns are compact both in space and time. The read-ahead logic
+ * tries to grant larger read-ahead size to better readers under the constraint
+ * of system memory and load pressure.
+ *
+ * It employs two methods to estimate the max thrashing safe read-ahead size:
+ * 1. state based - the default one
+ * 2. context based - the failsafe one
+ * The integration of the dual methods has the merit of being agile and robust.
+ * It makes the overall design clean: special cases are handled in general by
+ * the stateless method, leaving the stateful one simple and fast.
+ *
+ * To improve throughput and decrease read delay, the logic 'looks ahead'.
+ * In most read-ahead chunks, one page will be selected and tagged with
+ * PG_readahead. Later when the page with PG_readahead is read, the logic
+ * will be notified to submit the next read-ahead chunk in advance.
+ *
+ * a read-ahead chunk
+ * +-----------------------------------------+
+ * | # PG_readahead |
+ * +-----------------------------------------+
+ * ^ When this page is read, notify me for the next read-ahead.
+ *
+ */
+
+#ifdef CONFIG_ADAPTIVE_READAHEAD
+
+/*
+ * Move pages in danger (of thrashing) to the head of inactive_list.
+ * Not expected to happen frequently.
+ */
+static unsigned long rescue_pages(struct page *page, unsigned long nr_pages)
+{
+ int pgrescue = 0;
+ pgoff_t index = page_index(page);
+ struct address_space *mapping = page_mapping(page);
+ struct page *grabbed_page = NULL;
+ struct zone *zone;
+
+ dprintk("rescue_pages(ino=%lu, index=%lu nr=%lu)\n",
+ mapping->host->i_ino, index, nr_pages);
+
+ for(;;) {
+ zone = page_zone(page);
+ spin_lock_irq(&zone->lru_lock);
+
+ if (!PageLRU(page))
+ goto out_unlock;
+
+ while (page_mapping(page) == mapping &&
+ page_index(page) == index) {
+ struct page *the_page = page;
+ page = list_entry((page)->lru.prev, struct page, lru);
+ if (!PageActive(the_page) &&
+ !PageLocked(the_page) &&
+ page_count(the_page) == 1) {
+ list_move(&the_page->lru, &zone->inactive_list);
+ pgrescue++;
+ }
+ index++;
+ if (!--nr_pages)
+ goto out_unlock;
+ }
+
+ spin_unlock_irq(&zone->lru_lock);
+ cond_resched();
+
+ if (grabbed_page)
+ page_cache_release(grabbed_page);
+ grabbed_page = page = find_get_page(mapping, index);
+ if (!page)
+ goto out;
+ }
+
+out_unlock:
+ spin_unlock_irq(&zone->lru_lock);
+out:
+ if (grabbed_page)
+ page_cache_release(grabbed_page);
+ ra_account(NULL, RA_EVENT_READAHEAD_RESCUE, pgrescue);
+ return nr_pages;
+}
+
+#endif /* CONFIG_ADAPTIVE_READAHEAD */
+
+/*
* Read-ahead events accounting.
*/
#ifdef CONFIG_DEBUG_READAHEAD
--
-
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