Page cache miss tracing using ftrace on mm/filemap

This patch includes two trace events on generic_perform_write and
do_generic_file_read to check on the address_space mapping for the
pages to be accessed by the request.

Change-Id: Ib319b9b2c971b9e5c76645be6cfd995ef9465d77
Signed-off-by: Daniel Campello <campello@google.com>

Conflicts:
	include/linux/pagemap.h

include/linux/pagemap.h

@@ -254,6 +254,9 @@ typedef int filler_t(void *, struct page *);
 
 extern struct page * find_get_page(struct address_space *mapping,
 				pgoff_t index);
+pgoff_t page_cache_next_hole(struct address_space *mapping,
+                             pgoff_t index, unsigned long max_scan);
+
 extern struct page * find_lock_page(struct address_space *mapping,
 				pgoff_t index);
 extern struct page * find_or_create_page(struct address_space *mapping,

include/trace/events/filemap.h

@@ -52,6 +52,52 @@ DEFINE_EVENT(mm_filemap_op_page_cache, mm_filemap_add_to_page_cache,
 	TP_ARGS(page)
 	);
 
+DECLARE_EVENT_CLASS(mm_filemap_find_page_cache_miss,
+
+	TP_PROTO(struct file *file, loff_t pos, size_t count, int read),
+
+	TP_ARGS(file, pos, count, read),
+
+	TP_STRUCT__entry(
+		__array(char, path, MAX_FILTER_STR_VAL)
+		__field(char *, path_name)
+		__field(loff_t, pos)
+		__field(size_t, count)
+		__field(int, miss)
+	),
+
+	TP_fast_assign(
+		__entry->path_name = d_path(&file->f_path, __entry->path, MAX_FILTER_STR_VAL);
+		__entry->pos	= pos;
+		__entry->count	= count;
+		__entry->miss = 0;
+		if ((pos & ~PAGE_CACHE_MASK) || (count % PAGE_SIZE) || read) {
+			unsigned long ret;
+			rcu_read_lock();
+			ret = (count ? page_cache_next_hole(file->f_mapping,
+					(pos >> PAGE_CACHE_SHIFT), ((count - 1) >> PAGE_CACHE_SHIFT) + 1) : 0);
+			rcu_read_unlock();
+			__entry->miss = (ret >= (pos >> PAGE_CACHE_SHIFT) &&
+					ret <= ((pos + count - 1) >> PAGE_CACHE_SHIFT));
+		}
+	),
+
+	TP_printk("path_name %s pos %lld count %lu miss %s",
+		  __entry->path_name,
+		  __entry->pos, __entry->count,
+		  (__entry->miss ? "yes" : "no"))
+);
+
+DEFINE_EVENT(mm_filemap_find_page_cache_miss, mm_filemap_do_generic_file_read,
+	TP_PROTO(struct file *file, loff_t pos, size_t count, int read),
+	TP_ARGS(file, pos, count, read)
+	);
+
+DEFINE_EVENT(mm_filemap_find_page_cache_miss, mm_filemap_generic_perform_write,
+	TP_PROTO(struct file *file, loff_t pos, size_t count, int read),
+	TP_ARGS(file, pos, count, read)
+	);
+
 #endif /* _TRACE_FILEMAP_H */
 
 /* This part must be outside protection */

mm/filemap.c

@@ -683,6 +683,47 @@ int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 	}
 }
 
+/**
+ * page_cache_next_hole - find the next hole (not-present entry)
+ * @mapping: mapping
+ * @index: index
+ * @max_scan: maximum range to search
+ *
+ * Search the set [index, min(index+max_scan-1, MAX_INDEX)] for the
+ * lowest indexed hole.
+ *
+ * Returns: the index of the hole if found, otherwise returns an index
+ * outside of the set specified (in which case 'return - index >=
+ * max_scan' will be true). In rare cases of index wrap-around, 0 will
+ * be returned.
+ *
+ * page_cache_next_hole may be called under rcu_read_lock. However,
+ * like radix_tree_gang_lookup, this will not atomically search a
+ * snapshot of the tree at a single point in time. For example, if a
+ * hole is created at index 5, then subsequently a hole is created at
+ * index 10, page_cache_next_hole covering both indexes may return 10
+ * if called under rcu_read_lock.
+ */
+pgoff_t page_cache_next_hole(struct address_space *mapping,
+                             pgoff_t index, unsigned long max_scan)
+{
+        unsigned long i;
+
+        for (i = 0; i < max_scan; i++) {
+                struct page *page;
+
+                page = radix_tree_lookup(&mapping->page_tree, index);
+                if (!page || radix_tree_exceptional_entry(page))
+                        break;
+                index++;
+                if (index == 0)
+                        break;
+        }
+
+        return index;
+}
+EXPORT_SYMBOL(page_cache_next_hole);
+
 /**
  * find_get_page - find and get a page reference
  * @mapping: the address_space to search
@@ -1109,6 +1150,8 @@ static void do_generic_file_read(struct file *filp, loff_t *ppos,
 	unsigned int prev_offset;
 	int error;
 
+	trace_mm_filemap_do_generic_file_read(filp, *ppos, desc->count, 1);
+
 	index = *ppos >> PAGE_CACHE_SHIFT;
 	prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT;
 	prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1);
@@ -2315,6 +2358,8 @@ static ssize_t generic_perform_write(struct file *file,
 	ssize_t written = 0;
 	unsigned int flags = 0;
 
+	trace_mm_filemap_generic_perform_write(file, pos, iov_iter_count(i), 0);
+
 	/*
 	 * Copies from kernel address space cannot fail (NFSD is a big user).
 	 */