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Date: Tue, 12 May 2009 18:48:51 -0400
From: Theodore Ts'o <tytso@....edu>
To: Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc: Theodore Ts'o <tytso@....edu>
Subject: [PATCH 5/6] ext4: Add documentation to the ext4_*get_block* functions
This adds more docmentation 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@....edu>
---
fs/ext4/inode.c | 73 ++++++++++++++++++++++++++++++++++++-------------------
1 files changed, 48 insertions(+), 25 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 48c8f81..9431d50 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,
@@ -1994,6 +1999,12 @@ 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)
{
@@ -2025,8 +2036,8 @@ 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))
@@ -2318,8 +2329,9 @@ 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
@@ -2327,7 +2339,6 @@ 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)
@@ -2380,7 +2391,23 @@ 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;
@@ -2453,7 +2480,7 @@ 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 */
@@ -2478,11 +2505,10 @@ 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;
}
@@ -2794,7 +2820,7 @@ 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);
@@ -3102,12 +3128,10 @@ 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,
@@ -3159,7 +3183,7 @@ 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;
@@ -3244,9 +3268,8 @@ 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.rc4.1.g3e14.dirty
--
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