ext4: Add documentation to the ext4_*get_block* functions

This adds more documentation to various internal functions in
fs/ext4/inode.c, most notably ext4_ind_get_blocks(),
ext4_da_get_block_write(), ext4_da_get_block_prep(),
ext4_normal_get_block_write().

In addition, the static function ext4_normal_get_block_write() has
been renamed noalloc_get_block_write(), since it is used in many
places far beyond ext4_normal_writepage().

Plenty of warnings have been added to the noalloc_get_block_write()
function, since the way it is used is amazingly fragile.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 55 insertions, 31 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 8b7564dfacdf..fd5f27a9b81b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -892,6 +892,10 @@ err_out:
 }
 
 /*
+ * The ext4_ind_get_blocks() function handles non-extents inodes
+ * (i.e., using the traditional indirect/double-indirect i_blocks
+ * scheme) for ext4_get_blocks().
+ *
  * Allocation strategy is simple: if we have to allocate something, we will
  * have to go the whole way to leaf. So let's do it before attaching anything
  * to tree, set linkage between the newborn blocks, write them if sync is
@@ -909,10 +913,11 @@ err_out:
  * return = 0, if plain lookup failed.
  * return < 0, error case.
  *
- *
- * Need to be called with
- * down_read(&EXT4_I(inode)->i_data_sem) if not allocating file system block
- * (ie, create is zero). Otherwise down_write(&EXT4_I(inode)->i_data_sem)
+ * The ext4_ind_get_blocks() function should be called with
+ * down_write(&EXT4_I(inode)->i_data_sem) if allocating filesystem
+ * blocks (i.e., flags has EXT4_GET_BLOCKS_CREATE set) or
+ * down_read(&EXT4_I(inode)->i_data_sem) if not allocating file system
+ * blocks.
  */
 static int ext4_ind_get_blocks(handle_t *handle, struct inode *inode,
                                   ext4_lblk_t iblock, unsigned int maxblocks,
@@ -1152,8 +1157,8 @@ int ext4_get_blocks(handle_t *handle, struct inode *inode, sector_t block,
         clear_buffer_unwritten(bh);
 
         /*
-         * Try to see if we can get  the block without requesting
-         * for new file system block.
+         * Try to see if we can get the block without requesting a new
+         * file system block.
          */
         down_read((&EXT4_I(inode)->i_data_sem));
         if (EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL) {
@@ -2000,6 +2005,12 @@ static void ext4_print_free_blocks(struct inode *inode)
         return;
 }
 
+/*
+ * This function is used by mpage_da_map_blocks().  We separate it out
+ * as a separate function just to make life easier, and because
+ * mpage_da_map_blocks() used to be a generic function that took a
+ * get_block_t.
+ */
 static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
                                    struct buffer_head *bh_result)
 {
@@ -2031,8 +2042,8 @@ static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
 
         /*
          * Update on-disk size along with block allocation we don't
-         * use 'extend_disksize' as size may change within already
-         * allocated block -bzzz
+         * use EXT4_GET_BLOCKS_EXTEND_DISKSIZE as size may change
+         * within already allocated block -bzzz
          */
         disksize = ((loff_t) iblock + ret) << inode->i_blkbits;
         if (disksize > i_size_read(inode))
@@ -2338,8 +2349,9 @@ static int __mpage_da_writepage(struct page *page,
 }
 
 /*
- * this is a special callback for ->write_begin() only
- * it's intention is to return mapped block or reserve space
+ * This is a special get_blocks_t callback which is used by
+ * ext4_da_write_begin().  It will either return mapped block or
+ * reserve space for a single block.
  *
  * For delayed buffer_head we have BH_Mapped, BH_New, BH_Delay set.
  * We also have b_blocknr = -1 and b_bdev initialized properly
@@ -2347,7 +2359,6 @@ static int __mpage_da_writepage(struct page *page,
  * For unwritten buffer_head we have BH_Mapped, BH_New, BH_Unwritten set.
  * We also have b_blocknr = physicalblock mapping unwritten extent and b_bdev
  * initialized properly.
- *
  */
 static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
                                   struct buffer_head *bh_result, int create)
@@ -2400,7 +2411,23 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
         return ret;
 }
 
-static int ext4_normal_get_block_write(struct inode *inode, sector_t iblock,
+/*
+ * This function is used as a standard get_block_t calback function
+ * when there is no desire to allocate any blocks.  It is used as a
+ * callback function for block_prepare_write(), nobh_writepage(), and
+ * block_write_full_page().  These functions should only try to map a
+ * single block at a time.
+ *
+ * Since this function doesn't do block allocations even if the caller
+ * requests it by passing in create=1, it is critically important that
+ * any caller checks to make sure that any buffer heads are returned
+ * by this function are either all already mapped or marked for
+ * delayed allocation before calling nobh_writepage() or
+ * block_write_full_page().  Otherwise, b_blocknr could be left
+ * unitialized, and the page write functions will be taken by
+ * surprise.
+ */
+static int noalloc_get_block_write(struct inode *inode, sector_t iblock,
                                    struct buffer_head *bh_result, int create)
 {
         int ret = 0;
@@ -2419,10 +2446,11 @@ static int ext4_normal_get_block_write(struct inode *inode, sector_t iblock,
 }
 
 /*
- * get called vi ext4_da_writepages after taking page lock (have journal handle)
- * get called via journal_submit_inode_data_buffers (no journal handle)
- * get called via shrink_page_list via pdflush (no journal handle)
- * or grab_page_cache when doing write_begin (have journal handle)
+ * This function can get called via...
+ *   - ext4_da_writepages after taking page lock (have journal handle)
+ *   - journal_submit_inode_data_buffers (no journal handle)
+ *   - shrink_page_list via pdflush (no journal handle)
+ *   - grab_page_cache when doing write_begin (have journal handle)
  */
 static int ext4_da_writepage(struct page *page,
                                 struct writeback_control *wbc)
@@ -2473,7 +2501,7 @@ static int ext4_da_writepage(struct page *page,
                  * do block allocation here.
                  */
                 ret = block_prepare_write(page, 0, PAGE_CACHE_SIZE,
-                                                ext4_normal_get_block_write);
+                                          noalloc_get_block_write);
                 if (!ret) {
                         page_bufs = page_buffers(page);
                         /* check whether all are mapped and non delay */
@@ -2498,11 +2526,10 @@ static int ext4_da_writepage(struct page *page,
         }
 
         if (test_opt(inode->i_sb, NOBH) && ext4_should_writeback_data(inode))
-                ret = nobh_writepage(page, ext4_normal_get_block_write, wbc);
+                ret = nobh_writepage(page, noalloc_get_block_write, wbc);
         else
-                ret = block_write_full_page(page,
-                                                ext4_normal_get_block_write,
-                                                wbc);
+                ret = block_write_full_page(page, noalloc_get_block_write,
+                                            wbc);
 
         return ret;
 }
@@ -2814,7 +2841,7 @@ retry:
         *pagep = page;
 
         ret = block_write_begin(file, mapping, pos, len, flags, pagep, fsdata,
-                                                        ext4_da_get_block_prep);
+                                ext4_da_get_block_prep);
         if (ret < 0) {
                 unlock_page(page);
                 ext4_journal_stop(handle);
@@ -3122,12 +3149,10 @@ static int __ext4_normal_writepage(struct page *page,
         struct inode *inode = page->mapping->host;
 
         if (test_opt(inode->i_sb, NOBH))
-                return nobh_writepage(page,
-                                        ext4_normal_get_block_write, wbc);
+                return nobh_writepage(page, noalloc_get_block_write, wbc);
         else
-                return block_write_full_page(page,
-                                                ext4_normal_get_block_write,
-                                                wbc);
+                return block_write_full_page(page, noalloc_get_block_write,
+                                             wbc);
 }
 
 static int ext4_normal_writepage(struct page *page,
@@ -3179,7 +3204,7 @@ static int __ext4_journalled_writepage(struct page *page,
         int err;
 
         ret = block_prepare_write(page, 0, PAGE_CACHE_SIZE,
-                                        ext4_normal_get_block_write);
+                                  noalloc_get_block_write);
         if (ret != 0)
                 goto out_unlock;
 
@@ -3264,9 +3289,8 @@ static int ext4_journalled_writepage(struct page *page,
                  * really know unless we go poke around in the buffer_heads.
                  * But block_write_full_page will do the right thing.
                  */
-                return block_write_full_page(page,
-                                                ext4_normal_get_block_write,
-                                                wbc);
+                return block_write_full_page(page, noalloc_get_block_write,
+                                             wbc);
         }
 no_write:
         redirty_page_for_writepage(wbc, page);