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Subject[PATCH 04/16] readahead: some preparation
Some random changes that do not fit in elsewhere.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---
include/linux/mm.h | 7 +
mm/filemap.c | 4 +
mm/readahead.c | 198 ++++++++++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 200 insertions(+), 9 deletions(-)
--- linux.orig/include/linux/mm.h
+++ linux/include/linux/mm.h
@@ -1009,6 +1009,13 @@ void handle_ra_miss(struct address_space
struct file_ra_state *ra, pgoff_t offset);
unsigned long max_sane_readahead(unsigned long nr);

+#ifdef CONFIG_DEBUG_FS
+extern u32 readahead_debug_level;
+#define READAHEAD_DEBUG_LEVEL(n) (readahead_debug_level >= n)
+#else
+#define READAHEAD_DEBUG_LEVEL(n) (0)
+#endif
+
/* Do stack extension */
extern int expand_stack(struct vm_area_struct *vma, unsigned long address);
#ifdef CONFIG_IA64
--- linux.orig/mm/readahead.c
+++ linux/mm/readahead.c
@@ -15,13 +15,63 @@
#include <linux/backing-dev.h>
#include <linux/pagevec.h>

+/* The default max/min read-ahead pages. */
+#define KB(size) (((size)*1024 + PAGE_CACHE_SIZE-1) / PAGE_CACHE_SIZE)
+#define MAX_RA_PAGES KB(VM_MAX_READAHEAD)
+#define MIN_RA_PAGES KB(VM_MIN_READAHEAD)
+
+#define next_page(pg) (list_entry((pg)->lru.prev, struct page, lru))
+#define prev_page(pg) (list_entry((pg)->lru.next, struct page, lru))
+
+/*
+ * Debug facilities.
+ */
+#ifdef CONFIG_DEBUG_FS
+#define DEBUG_READAHEAD
+#endif
+
+#ifdef DEBUG_READAHEAD
+#define dprintk(args...) \
+ do { if (READAHEAD_DEBUG_LEVEL(1)) printk(KERN_DEBUG args); } while(0)
+#define ddprintk(args...) \
+ do { if (READAHEAD_DEBUG_LEVEL(2)) printk(KERN_DEBUG args); } while(0)
+#else
+#define dprintk(args...) do { } while(0)
+#define ddprintk(args...) do { } while(0)
+#endif
+
+#ifdef DEBUG_READAHEAD
+#include <linux/jiffies.h>
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+#include <linux/init.h>
+
+u32 readahead_debug_level = 0;
+
+static int __init readahead_init(void)
+{
+ struct dentry *root;
+
+ root = debugfs_create_dir("readahead", NULL);
+
+ debugfs_create_u32("debug_level", 0644, root, &readahead_debug_level);
+
+ return 0;
+}
+
+module_init(readahead_init)
+#else /* !DEBUG_READAHEAD */
+
+#endif /* DEBUG_READAHEAD */
+
+
void default_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
{
}
EXPORT_SYMBOL(default_unplug_io_fn);

struct backing_dev_info default_backing_dev_info = {
- .ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE,
+ .ra_pages = MAX_RA_PAGES,
.state = 0,
.capabilities = BDI_CAP_MAP_COPY,
.unplug_io_fn = default_unplug_io_fn,
@@ -50,7 +100,7 @@ static inline unsigned long get_max_read

static inline unsigned long get_min_readahead(struct file_ra_state *ra)
{
- return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+ return MIN_RA_PAGES;
}

static inline void ra_off(struct file_ra_state *ra)
@@ -258,10 +308,11 @@ out:
*/
static int
__do_page_cache_readahead(struct address_space *mapping, struct file *filp,
- pgoff_t offset, unsigned long nr_to_read)
+ pgoff_t offset, unsigned long nr_to_read,
+ unsigned long lookahead_size)
{
struct inode *inode = mapping->host;
- struct page *page;
+ struct page *page = NULL;
unsigned long end_index; /* The last page we want to read */
LIST_HEAD(page_pool);
int page_idx;
@@ -271,7 +322,7 @@ __do_page_cache_readahead(struct address
if (isize == 0)
goto out;

- end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+ end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);

/*
* Preallocate as many pages as we will need.
@@ -284,8 +335,14 @@ __do_page_cache_readahead(struct address
break;

page = radix_tree_lookup(&mapping->page_tree, page_offset);
- if (page)
+ if (page) {
+#ifdef READAHEAD_STREAMING
+ if (prefer_adaptive_readahead() &&
+ page_idx == nr_to_read - lookahead_size)
+ SetPageReadahead(page);
+#endif
continue;
+ }

read_unlock_irq(&mapping->tree_lock);
page = page_cache_alloc_cold(mapping);
@@ -294,6 +351,9 @@ __do_page_cache_readahead(struct address
break;
page->index = page_offset;
list_add(&page->lru, &page_pool);
+ if (prefer_adaptive_readahead() &&
+ page_idx == nr_to_read - lookahead_size)
+ SetPageReadahead(page);
ret++;
}
read_unlock_irq(&mapping->tree_lock);
@@ -330,7 +390,7 @@ int force_page_cache_readahead(struct ad
if (this_chunk > nr_to_read)
this_chunk = nr_to_read;
err = __do_page_cache_readahead(mapping, filp,
- offset, this_chunk);
+ offset, this_chunk, 0);
if (err < 0) {
ret = err;
break;
@@ -377,7 +437,7 @@ int do_page_cache_readahead(struct addre
if (bdi_read_congested(mapping->backing_dev_info))
return -1;

- return __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+ return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
}

/*
@@ -397,7 +457,10 @@ blockable_page_cache_readahead(struct ad
if (!block && bdi_read_congested(mapping->backing_dev_info))
return 0;

- actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read);
+ actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0);
+
+ dprintk("blockable-readahead(ino=%lu, ra=%lu+%lu) = %d\n",
+ mapping->host->i_ino, offset, nr_to_read, actual);

return check_ra_success(ra, nr_to_read, actual);
}
@@ -575,3 +638,120 @@ unsigned long max_sane_readahead(unsigne
__get_zone_counts(&active, &inactive, &free, NODE_DATA(numa_node_id()));
return min(nr, (inactive + free) / 2);
}
+
+/*
+ * 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 pressures.
+ *
+ * It employs two methods to estimate the max thrashing safe read-ahead size:
+ * 1. state based - the default one
+ * 2. context based - the fail safe 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.
+ *
+ *
+ * Here are some variable names used frequently:
+ *
+ * |<------- la_size ------>|
+ * +-----------------------------------------+
+ * | # |
+ * +-----------------------------------------+
+ * ra_index -->|<---------------- ra_size -------------->|
+ *
+ */
+
+/*
+ * The nature of read-ahead allows most tests to fail or even be wrong.
+ * Here we just do not bother to call get_page(), it's meaningless anyway.
+ */
+static inline struct page *__find_page(struct address_space *mapping,
+ pgoff_t offset)
+{
+ return radix_tree_lookup(&mapping->page_tree, offset);
+}
+
+static inline struct page *find_page(struct address_space *mapping,
+ pgoff_t offset)
+{
+ struct page *page;
+
+ read_lock_irq(&mapping->tree_lock);
+ page = __find_page(mapping, offset);
+ read_unlock_irq(&mapping->tree_lock);
+#ifdef DEBUG_READAHEAD_RADIXTREE
+ if (page)
+ BUG_ON(page->index != offset);
+#endif
+ return page;
+}
+
+/*
+ * Move pages in danger (of thrashing) to the head of inactive_list.
+ * Not expected to happen frequently.
+ */
+static int rescue_pages(struct page *page, pgoff_t nr_pages)
+{
+ unsigned long pgrescue;
+ pgoff_t index;
+ struct address_space *mapping;
+ struct zone *zone;
+
+ BUG_ON(!nr_pages || !page);
+ pgrescue = 0;
+ index = page_index(page);
+ mapping = page_mapping(page);
+
+ 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 = next_page(page);
+ if (!PageActive(the_page) &&
+ !PageActivate(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);
+
+ page = find_page(mapping, index);
+ if (!page)
+ goto out;
+ }
+out_unlock:
+ spin_unlock_irq(&zone->lru_lock);
+out:
+ ra_account(0, RA_EVENT_READAHEAD_RESCUE, pgrescue);
+
+ return nr_pages ? index : 0;
+}
--- linux.orig/mm/filemap.c
+++ linux/mm/filemap.c
@@ -780,6 +780,10 @@ void do_generic_mapping_read(struct addr
if (!isize)
goto out;

+ if (READAHEAD_DEBUG_LEVEL(5))
+ printk(KERN_DEBUG "read-file(ino=%lu, req=%lu+%lu)\n",
+ inode->i_ino, index, last_index - index);
+
end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
for (;;) {
struct page *page;
--
-
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