[PATCH 32/49] ext4: Add documentation to the ext4_*get_block* functions

Theodore Ts'o
2009-06-08T19:28:27Z

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>
---
fs/ext4/inode.c | 86 +++++++++++++++++++++++++++++++++++--------------------
1 files changed, 55 insertions(+), 31 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 8b7564d..fd5f27a 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);
--
1.6.3.2.1.gb9f7d.dirty