ext4: add comments to definition of ext4_io_end_t

This should make it more clear what this structure is used
for, and how some of the (mutually exclusive) fields are
used to keep page cache references.

Signed-off-by: Curt Wohlgemuth <curtw@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 10 insertions, 3 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 37e7d8b66c99..26ee54b2ce82 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -194,18 +194,25 @@ struct ext4_io_page {
 
 #define MAX_IO_PAGES 128
 
+/*
+ * For converting uninitialized extents on a work queue.
+ *
+ * 'page' is only used from the writepage() path; 'pages' is only used for
+ * buffered writes; they are used to keep page references until conversion
+ * takes place.  For AIO/DIO, neither field is filled in.
+ */
 typedef struct ext4_io_end {
         struct list_head        list;           /* per-file finished IO list */
         struct inode            *inode;         /* file being written to */
         unsigned int            flag;           /* unwritten or not */
-        struct page             *page;          /* page struct for buffer write */
+        struct page             *page;          /* for writepage() path */
         loff_t                  offset;         /* offset in the file */
         ssize_t                 size;           /* size of the extent */
         struct work_struct      work;           /* data work queue */
         struct kiocb            *iocb;          /* iocb struct for AIO */
         int                     result;         /* error value for AIO */
-        int                     num_io_pages;
-        struct ext4_io_page     *pages[MAX_IO_PAGES];
+        int                     num_io_pages;   /* for writepages() */
+        struct ext4_io_page     *pages[MAX_IO_PAGES]; /* for writepages() */
 } ext4_io_end_t;
 
 struct ext4_io_submit {