[PATCH] readahead commentary

Add a few comments surrounding the generic readahead API.

Also convert some ulongs into pgoff_t: the identifier for PAGE_CACHE_SIZE
offsets into pagecache.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 26 insertions, 13 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5c1fb0a2e806..7b115feca4df 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -932,13 +932,13 @@ int write_one_page(struct page *page, int wait);
                                          * turning readahead off */
 
 int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                        unsigned long offset, unsigned long nr_to_read);
+                        pgoff_t offset, unsigned long nr_to_read);
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                        unsigned long offset, unsigned long nr_to_read);
-unsigned long  page_cache_readahead(struct address_space *mapping,
+                        pgoff_t offset, unsigned long nr_to_read);
+unsigned long page_cache_readahead(struct address_space *mapping,
                           struct file_ra_state *ra,
                           struct file *filp,
-                          unsigned long offset,
+                          pgoff_t offset,
                           unsigned long size);
 void handle_ra_miss(struct address_space *mapping, 
                     struct file_ra_state *ra, pgoff_t offset);
diff --git a/mm/readahead.c b/mm/readahead.c
index d0b50034e245..72e7adbb87c7 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -254,7 +254,7 @@ out:
  */
 static int
 __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                        unsigned long offset, unsigned long nr_to_read)
+                        pgoff_t offset, unsigned long nr_to_read)
 {
         struct inode *inode = mapping->host;
         struct page *page;
@@ -274,7 +274,7 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
          */
         read_lock_irq(&mapping->tree_lock);
         for (page_idx = 0; page_idx < nr_to_read; page_idx++) {
-                unsigned long page_offset = offset + page_idx;
+                pgoff_t page_offset = offset + page_idx;
                 
                 if (page_offset > end_index)
                         break;
@@ -311,7 +311,7 @@ out:
  * memory at once.
  */
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                unsigned long offset, unsigned long nr_to_read)
+                pgoff_t offset, unsigned long nr_to_read)
 {
         int ret = 0;
 
@@ -368,7 +368,7 @@ static inline int check_ra_success(struct file_ra_state *ra,
  * request queues.
  */
 int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                        unsigned long offset, unsigned long nr_to_read)
+                        pgoff_t offset, unsigned long nr_to_read)
 {
         if (bdi_read_congested(mapping->backing_dev_info))
                 return -1;
@@ -385,7 +385,7 @@ int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
  */
 static int
 blockable_page_cache_readahead(struct address_space *mapping, struct file *filp,
-                        unsigned long offset, unsigned long nr_to_read,
+                        pgoff_t offset, unsigned long nr_to_read,
                         struct file_ra_state *ra, int block)
 {
         int actual;
@@ -430,14 +430,27 @@ static int make_ahead_window(struct address_space *mapping, struct file *filp,
         return ret;
 }
 
-/*
- * page_cache_readahead is the main function.  If performs the adaptive
+/**
+ * page_cache_readahead - generic adaptive readahead
+ * @mapping: address_space which holds the pagecache and I/O vectors
+ * @ra: file_ra_state which holds the readahead state
+ * @filp: passed on to ->readpage() and ->readpages()
+ * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
+ * @req_size: hint: total size of the read which the caller is performing in
+ *            PAGE_CACHE_SIZE units
+ *
+ * page_cache_readahead() is the main function.  If performs the adaptive
  * readahead window size management and submits the readahead I/O.
+ *
+ * Note that @filp is purely used for passing on to the ->readpage[s]()
+ * handler: it may refer to a different file from @mapping (so we may not use
+ * @filp->f_mapping or @filp->f_dentry->d_inode here).
+ * Also, @ra may not be equal to &@filp->f_ra.
+ *
  */
 unsigned long
 page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
-                     struct file *filp, unsigned long offset,
-                     unsigned long req_size)
+                     struct file *filp, pgoff_t offset, unsigned long req_size)
 {
         unsigned long max, newsize;
         int sequential;