[PATCH] add stats for page reclaim via inode freeing

pagecache can be reclaimed via the page LRU and via prune_icache.  We
currently don't know how much reclaim is happening via each.

The patch adds instrumentation to display the number of pages which were
freed via prune_icache.  This is displayed in /proc/vmstat:pginodesteal and
/proc/vmstat:kswapd_inodesteal.

Turns out that under some workloads (well, dbench at least), fully half of
page reclaim is via the unused inode list.  Which seems quite OK to me.

fs/inode.c

@@ -16,6 +16,7 @@
 #include <linux/backing-dev.h>
 #include <linux/wait.h>
 #include <linux/hash.h>
+#include <linux/swap.h>
 #include <linux/security.h>
 
 /*
@@ -392,6 +393,7 @@ static void prune_icache(int nr_to_scan)
 	LIST_HEAD(freeable);
 	int nr_pruned = 0;
 	int nr_scanned;
+	unsigned long reap = 0;
 
 	spin_lock(&inode_lock);
 	for (nr_scanned = 0; nr_scanned < nr_to_scan; nr_scanned++) {
@@ -410,7 +412,7 @@ static void prune_icache(int nr_to_scan)
 			__iget(inode);
 			spin_unlock(&inode_lock);
 			if (remove_inode_buffers(inode))
-				invalidate_inode_pages(&inode->i_data);
+				reap += invalidate_inode_pages(&inode->i_data);
 			iput(inode);
 			spin_lock(&inode_lock);
 
@@ -428,6 +430,10 @@ static void prune_icache(int nr_to_scan)
 	inodes_stat.nr_unused -= nr_pruned;
 	spin_unlock(&inode_lock);
 	dispose_list(&freeable);
+	if (current_is_kswapd)
+		mod_page_state(kswapd_inodesteal, reap);
+	else
+		mod_page_state(pginodesteal, reap);
 }
 
 /*

include/linux/fs.h

@@ -1102,9 +1102,9 @@ extern int full_check_disk_change(struct block_device *);
 extern int __check_disk_change(dev_t);
 extern int invalidate_inodes(struct super_block *);
 extern int invalidate_device(kdev_t, int);
-extern void invalidate_mapping_pages(struct address_space *mapping,
+unsigned long invalidate_mapping_pages(struct address_space *mapping,
 					pgoff_t start, pgoff_t end);
-extern void invalidate_inode_pages(struct address_space *mapping);
+unsigned long invalidate_inode_pages(struct address_space *mapping);
 extern void invalidate_inode_pages2(struct address_space *mapping);
 extern void write_inode_now(struct inode *, int);
 extern int filemap_fdatawrite(struct address_space *);

include/linux/page-flags.h

@@ -97,15 +97,20 @@ struct page_state {
 	unsigned long pswpin;		/* swap reads */
 	unsigned long pswpout;		/* swap writes */
 	unsigned long pgalloc;		/* page allocations */
+
 	unsigned long pgfree;		/* page freeings */
 	unsigned long pgactivate;	/* pages moved inactive->active */
 	unsigned long pgdeactivate;	/* pages moved active->inactive */
 	unsigned long pgfault;		/* faults (major+minor) */
 	unsigned long pgmajfault;	/* faults (major only) */
+
 	unsigned long pgscan;		/* pages scanned by page reclaim */
 	unsigned long pgrefill;		/* inspected in refill_inactive_zone */
 	unsigned long pgsteal;		/* total pages reclaimed */
+	unsigned long pginodesteal;	/* pages reclaimed via inode freeing */
 	unsigned long kswapd_steal;	/* pages reclaimed by kswapd */
+
+	unsigned long kswapd_inodesteal;/* reclaimed via kswapd inode freeing */
 	unsigned long pageoutrun;	/* kswapd's calls to page reclaim */
 	unsigned long allocstall;	/* direct reclaim calls */
 	unsigned long pgrotated;	/* pages rotated to tail of the LRU */

mm/page_alloc.c

@@ -1379,15 +1379,20 @@ static char *vmstat_text[] = {
 	"pswpin",
 	"pswpout",
 	"pgalloc",
+
 	"pgfree",
 	"pgactivate",
 	"pgdeactivate",
 	"pgfault",
 	"pgmajfault",
+
 	"pgscan",
 	"pgrefill",
 	"pgsteal",
+	"pginodesteal",
 	"kswapd_steal",
+
+	"kswapd_inodesteal",
 	"pageoutrun",
 	"allocstall",
 	"pgrotated",

mm/truncate.c

@@ -64,24 +64,25 @@ truncate_complete_page(struct address_space *mapping, struct page *page)
  * ->page_lock.  That provides exclusion against the __set_page_dirty
  * functions.
  */
-static void
+static int
 invalidate_complete_page(struct address_space *mapping, struct page *page)
 {
 	if (page->mapping != mapping)
-		return;
+		return 0;
 
 	if (PagePrivate(page) && !try_to_release_page(page, 0))
-		return;
+		return 0;
 
 	write_lock(&mapping->page_lock);
 	if (PageDirty(page)) {
 		write_unlock(&mapping->page_lock);
-	} else {
+		return 0;
+	}
 	__remove_from_page_cache(page);
 	write_unlock(&mapping->page_lock);
 	ClearPageUptodate(page);
 	page_cache_release(page);	/* pagecache ref */
-	}
+	return 1;
 }
 
 /**
@@ -189,11 +190,12 @@ void truncate_inode_pages(struct address_space *mapping, loff_t lstart)
  * invalidate pages which are dirty, locked, under writeback or mapped into
  * pagetables.
  */
-void invalidate_mapping_pages(struct address_space *mapping,
+unsigned long invalidate_mapping_pages(struct address_space *mapping,
 				pgoff_t start, pgoff_t end)
 {
 	struct pagevec pvec;
 	pgoff_t next = start;
+	unsigned long ret = 0;
 	int i;
 
 	pagevec_init(&pvec, 0);
@@ -213,18 +215,19 @@ void invalidate_mapping_pages(struct address_space *mapping,
 				goto unlock;
 			if (page_mapped(page))
 				goto unlock;
-			invalidate_complete_page(mapping, page);
+			ret += invalidate_complete_page(mapping, page);
 unlock:
 			unlock_page(page);
 		}
 		pagevec_release(&pvec);
 		cond_resched();
 	}
+	return ret;
 }
 
-void invalidate_inode_pages(struct address_space *mapping)
+unsigned long invalidate_inode_pages(struct address_space *mapping)
 {
-	invalidate_mapping_pages(mapping, 0, ~0UL);
+	return invalidate_mapping_pages(mapping, 0, ~0UL);
 }
 
 /**