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Subject[PATCH 06/12] readahead: context based method
This is the slow code path.

No valid state info is available, so the page cache is queried to abtain the
required position/timing infomation.

Major steps:
- look back/forward to find the ra_index;
- look back to estimate a thrashing safe ra_size;
- assemble the next read-ahead request in file_ra_state;
- submit it.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
mm/readahead.c | 362 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 362 insertions(+)
--- linux.orig/mm/readahead.c
+++ linux/mm/readahead.c
@@ -1166,6 +1166,368 @@ state_based_readahead(struct address_spa
return ra_dispatch(ra, mapping, filp);
}

+/*
+ * Page cache context based estimation of read-ahead/look-ahead size/index.
+ *
+ * The logic first looks around to find the start point of next read-ahead,
+ * and then, if necessary, looks backward in the inactive_list to get an
+ * estimation of the thrashing-threshold.
+ *
+ * The estimation theory can be illustrated with figure:
+ *
+ * chunk A chunk B chunk C head
+ *
+ * l01 l11 l12 l21 l22
+ *| |-->|-->| |------>|-->| |------>|
+ *| +-------+ +-----------+ +-------------+ |
+ *| | # | | # | | # | |
+ *| +-------+ +-----------+ +-------------+ |
+ *| |<==============|<===========================|<============================|
+ * L0 L1 L2
+ *
+ * Let f(l) = L be a map from
+ * l: the number of pages read by the stream
+ * to
+ * L: the number of pages pushed into inactive_list in the mean time
+ * then
+ * f(l01) <= L0
+ * f(l11 + l12) = L1
+ * f(l21 + l22) = L2
+ * ...
+ * f(l01 + l11 + ...) <= Sum(L0 + L1 + ...)
+ * <= Length(inactive_list) = f(thrashing-threshold)
+ *
+ * So the count of countinuous history pages left in the inactive_list is always
+ * a lower estimation of the true thrashing-threshold.
+ */
+
+#define PAGE_REFCNT_0 0
+#define PAGE_REFCNT_1 (1 << PG_referenced)
+#define PAGE_REFCNT_2 (1 << PG_active)
+#define PAGE_REFCNT_3 ((1 << PG_active) | (1 << PG_referenced))
+#define PAGE_REFCNT_MASK PAGE_REFCNT_3
+
+/*
+ * STATUS REFERENCE COUNT
+ * __ 0
+ * _R PAGE_REFCNT_1
+ * A_ PAGE_REFCNT_2
+ * AR PAGE_REFCNT_3
+ *
+ * A/R: Active / Referenced
+ */
+static inline unsigned long page_refcnt(struct page *page)
+{
+ return page->flags & PAGE_REFCNT_MASK;
+}
+
+/*
+ * STATUS REFERENCE COUNT TYPE
+ * __ 0 not in inactive list
+ * __ 0 fresh
+ * _R PAGE_REFCNT_1 stale
+ * A_ PAGE_REFCNT_2 disturbed once
+ * AR PAGE_REFCNT_3 disturbed twice
+ *
+ * A/R: Active / Referenced
+ */
+static inline unsigned long cold_page_refcnt(struct page *page)
+{
+ if (!page || PageActive(page))
+ return 0;
+
+ return page_refcnt(page);
+}
+
+static inline char page_refcnt_symbol(struct page *page)
+{
+ if (!page)
+ return 'X';
+ switch (page_refcnt(page)) {
+ case 0:
+ return '_';
+ case PAGE_REFCNT_1:
+ return '-';
+ case PAGE_REFCNT_2:
+ return '=';
+ case PAGE_REFCNT_3:
+ return '#';
+ }
+ return '?';
+}
+
+/*
+ * Count/estimate cache hits in range [first_index, last_index].
+ * The estimation is simple and optimistic.
+ */
+static int count_cache_hit(struct address_space *mapping,
+ pgoff_t first_index, pgoff_t last_index)
+{
+ struct page *page;
+ int size = last_index - first_index + 1;
+ int count = 0;
+ int i;
+
+ read_lock_irq(&mapping->tree_lock);
+
+ /*
+ * The first page may well is chunk head and has been accessed,
+ * so it is index 0 that makes the estimation optimistic. This
+ * behavior guarantees a readahead when (size < ra_max) and
+ * (readahead_hit_rate >= 16).
+ */
+ for (i = 0; i < 16;) {
+ page = __find_page(mapping, first_index +
+ size * ((i++ * 29) & 15) / 16);
+ if (cold_page_refcnt(page) >= PAGE_REFCNT_1 && ++count >= 2)
+ break;
+ }
+
+ read_unlock_irq(&mapping->tree_lock);
+
+ return size * count / i;
+}
+
+/*
+ * Look back and check history pages to estimate thrashing-threshold.
+ */
+static int query_page_cache(struct address_space *mapping,
+ struct file_ra_state *ra,
+ unsigned long *remain, pgoff_t offset,
+ unsigned long ra_min, unsigned long ra_max)
+{
+ int count;
+ pgoff_t index;
+ unsigned long nr_lookback;
+ struct radix_tree_cache cache;
+
+ /*
+ * Scan backward and check the near @ra_max pages.
+ * The count here determines ra_size.
+ */
+ read_lock_irq(&mapping->tree_lock);
+ index = radix_tree_lookup_head(&mapping->page_tree, offset, ra_max);
+ read_unlock_irq(&mapping->tree_lock);
+#ifdef DEBUG_READAHEAD_RADIXTREE
+ if (index <= offset) {
+ WARN_ON(!find_page(mapping, index));
+ if (index + ra_max > offset)
+ WARN_ON(find_page(mapping, index - 1));
+ } else {
+ BUG_ON(index > offset + 1);
+ WARN_ON(find_page(mapping, offset));
+ }
+#endif
+
+ *remain = offset - index + 1;
+
+ if (unlikely(*remain <= ra_min))
+ return ra_min;
+
+ if (!index)
+ return *remain;
+
+ if (offset + 1 == ra->readahead_index && ra_cache_hit_ok(ra))
+ count = *remain;
+ else if (count_cache_hit(mapping, index, offset) *
+ readahead_hit_rate >= *remain)
+ count = *remain;
+ else
+ return ra_min;
+
+ if (count < ra_max)
+ goto out;
+
+ /*
+ * Check the far pages coarsely.
+ * The big count here helps increase la_size.
+ */
+ nr_lookback = ra_max * (LOOKAHEAD_RATIO + 1) *
+ 100 / (readahead_ratio + 1);
+ if (nr_lookback > offset)
+ nr_lookback = offset;
+
+ radix_tree_cache_init(&cache);
+ read_lock_irq(&mapping->tree_lock);
+ for (count += ra_max; count < nr_lookback; count += ra_max) {
+ struct radix_tree_node *node;
+ node = radix_tree_cache_lookup_node(&mapping->page_tree,
+ &cache, offset - count, 1);
+ if (!node)
+ break;
+#ifdef DEBUG_READAHEAD_RADIXTREE
+ if (node != radix_tree_lookup_node(&mapping->page_tree,
+ offset - count, 1)) {
+ read_unlock_irq(&mapping->tree_lock);
+ printk(KERN_ERR "check radix_tree_cache_lookup_node!\n");
+ return 1;
+ }
+#endif
+ }
+ read_unlock_irq(&mapping->tree_lock);
+
+ /*
+ * For sequential read that extends from index 0, the counted value
+ * may well be far under the true threshold, so return it unmodified
+ * for further process in adjust_rala_accelerated().
+ */
+ if (count >= offset)
+ return offset + 1;
+
+out:
+ count = count * readahead_ratio / 100;
+ return count;
+}
+
+/*
+ * Scan backward in the file for the first non-present page.
+ */
+static inline pgoff_t first_absent_page_bw(struct address_space *mapping,
+ pgoff_t index, unsigned long max_scan)
+{
+ struct radix_tree_cache cache;
+ struct page *page;
+ pgoff_t origin;
+
+ origin = index;
+ if (max_scan > index)
+ max_scan = index;
+
+ radix_tree_cache_init(&cache);
+ read_lock_irq(&mapping->tree_lock);
+ for (; origin - index <= max_scan;) {
+ page = radix_tree_cache_lookup(&mapping->page_tree,
+ &cache, --index);
+ if (page) {
+ index++;
+ break;
+ }
+ }
+ read_unlock_irq(&mapping->tree_lock);
+
+ return index;
+}
+
+/*
+ * Scan forward in the file for the first non-present page.
+ */
+static inline pgoff_t first_absent_page(struct address_space *mapping,
+ pgoff_t index, unsigned long max_scan)
+{
+ pgoff_t ra_index;
+
+ read_lock_irq(&mapping->tree_lock);
+ ra_index = radix_tree_lookup_tail(&mapping->page_tree,
+ index + 1, max_scan);
+ read_unlock_irq(&mapping->tree_lock);
+
+#ifdef DEBUG_READAHEAD_RADIXTREE
+ BUG_ON(ra_index <= index);
+ if (index + max_scan > index) {
+ if (ra_index <= index + max_scan)
+ WARN_ON(find_page(mapping, ra_index));
+ WARN_ON(!find_page(mapping, ra_index - 1));
+ }
+#endif
+
+ if (ra_index <= index + max_scan)
+ return ra_index;
+ else
+ return 0;
+}
+
+/*
+ * Determine the request parameters for context based read-ahead that extends
+ * from start of file.
+ *
+ * The major weakness of stateless method is perhaps the slow grow up speed of
+ * ra_size. The logic tries to make up for this in the important case of
+ * sequential reads that extend from start of file. In this case, the ra_size
+ * is not chosen to make the whole next chunk safe (as in normal ones). Only
+ * half of which is safe. The added 'unsafe' half is the look-ahead part. It
+ * is expected to be safeguarded by rescue_pages() when the previous chunks are
+ * lost.
+ */
+static inline int adjust_rala_accelerated(unsigned long ra_max,
+ unsigned long *ra_size, unsigned long *la_size)
+{
+ pgoff_t index = *ra_size;
+
+ *ra_size -= min(*ra_size, *la_size);
+ *ra_size = *ra_size * readahead_ratio / 100;
+ *la_size = index * readahead_ratio / 100;
+ *ra_size += *la_size;
+
+ if (*ra_size > ra_max)
+ *ra_size = ra_max;
+ if (*la_size > *ra_size)
+ *la_size = *ra_size;
+
+ return 1;
+}
+
+/*
+ * Main function for page context based read-ahead.
+ */
+static inline int
+try_context_based_readahead(struct address_space *mapping,
+ struct file_ra_state *ra,
+ struct page *prev_page, struct page *page,
+ pgoff_t index, unsigned long ra_size,
+ unsigned long ra_min, unsigned long ra_max)
+{
+ pgoff_t ra_index;
+ unsigned long la_size;
+ unsigned long remain_pages;
+
+ /* Where to start read-ahead?
+ * NFSv3 daemons may process adjacent requests in parallel,
+ * leading to many locally disordered, globally sequential reads.
+ * So do not require nearby history pages to be present or accessed.
+ */
+ if (page) {
+ ra_index = first_absent_page(mapping, index, ra_max * 5 / 4);
+ if (unlikely(!ra_index))
+ return -1;
+ } else if (!prev_page) {
+ ra_index = first_absent_page_bw(mapping, index,
+ readahead_hit_rate + ra_min);
+ if (index - ra_index > readahead_hit_rate + ra_min)
+ return 0;
+ ra_min += 2 * (index - ra_index);
+ index = ra_index;
+ } else {
+ ra_index = index;
+ if (ra_has_index(ra, index))
+ ra_account(ra, RA_EVENT_READAHEAD_MUTILATE,
+ ra->readahead_index - index);
+ }
+
+ ra_size = query_page_cache(mapping, ra, &remain_pages,
+ index - 1, ra_min, ra_max);
+
+ la_size = ra_index - index;
+ if (remain_pages <= la_size && la_size > 1) {
+ rescue_pages(page, la_size);
+ return -1;
+ }
+
+ if (ra_size == index) {
+ if (!adjust_rala_accelerated(ra_max, &ra_size, &la_size))
+ return -1;
+ ra_set_class(ra, RA_CLASS_CONTEXT_ACCELERATED);
+ } else {
+ if (!adjust_rala(ra_max, &ra_size, &la_size))
+ return -1;
+ ra_set_class(ra, RA_CLASS_CONTEXT);
+ }
+
+ ra_set_index(ra, index, ra_index);
+ ra_set_size(ra, ra_size, la_size);
+
+ return 1;
+}
+

/*
* ra_size is mainly determined by:
--
-
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