[PATCH 03/49] ext4: Mark the unwritten buffer_head as mapped during write_begin

Theodore Ts'o
2009-06-08T19:25:13Z

From: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

Setting BH_Unwritten buffer_heads as BH_Mapped avoids multiple
(unnecessary) calls to get_block() during the call to the write(2)
system call. Setting BH_Unwritten buffer heads as BH_Mapped requires
that the writepages() functions can handle BH_Unwritten buffer_heads.

After this commit, things work as follows:

ext4_ext_get_block() returns unmapped, unwritten, buffer head when
called with create = 0 for prealloc space. This makes sure we handle
the read path and non-delayed allocation case correctly. Even though
the buffer head is marked unmapped we have valid b_blocknr and b_bdev
values in the buffer_head.

ext4_da_get_block_prep() called for block resrevation will now return
mapped, unwritten, new buffer_head for prealloc space. This avoids
multiple calls to get_block() for write to same offset. By making such
buffers as BH_New, we also assure that sub-block zeroing of buffered
writes happens correctly.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
fs/ext4/extents.c | 4 +-
fs/ext4/inode.c | 82 +++++++++++++++++++++++++++++++++-------------------
2 files changed, 54 insertions(+), 32 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index a953214..ea5c476 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2872,6 +2872,8 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
if (create == EXT4_CREATE_UNINITIALIZED_EXT)
goto out;
if (!create) {
+ if (allocated > max_blocks)
+ allocated = max_blocks;
/*
* We have blocks reserved already. We
* return allocated blocks so that delalloc
@@ -2879,8 +2881,6 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
* the buffer head will be unmapped so that
* a read from the block returns 0s.
*/
- if (allocated > max_blocks)
- allocated = max_blocks;
set_buffer_unwritten(bh_result);
bh_result->b_bdev = inode->i_sb->s_bdev;
bh_result->b_blocknr = newblock;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d7ad0bb..96f3366 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1852,7 +1852,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
* @logical - first logical block to start assignment with
*
* the function goes through all passed space and put actual disk
- * block numbers into buffer heads, dropping BH_Delay
+ * block numbers into buffer heads, dropping BH_Delay and BH_Unwritten
*/
static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
struct buffer_head *exbh)
@@ -1902,16 +1902,24 @@ static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
do {
if (cur_logical >= logical + blocks)
break;
- if (buffer_delay(bh)) {
- bh->b_blocknr = pblock;
- clear_buffer_delay(bh);
- bh->b_bdev = inode->i_sb->s_bdev;
- } else if (buffer_unwritten(bh)) {
- bh->b_blocknr = pblock;
- clear_buffer_unwritten(bh);
- set_buffer_mapped(bh);
- set_buffer_new(bh);
- bh->b_bdev = inode->i_sb->s_bdev;
+
+ if (buffer_delay(bh) ||
+ buffer_unwritten(bh)) {
+
+ BUG_ON(bh->b_bdev != inode->i_sb->s_bdev);
+
+ if (buffer_delay(bh)) {
+ clear_buffer_delay(bh);
+ bh->b_blocknr = pblock;
+ } else {
+ /*
+ * unwritten already should have
+ * blocknr assigned. Verify that
+ */
+ clear_buffer_unwritten(bh);
+ BUG_ON(bh->b_blocknr != pblock);
+ }
+
} else if (buffer_mapped(bh))
BUG_ON(bh->b_blocknr != pblock);

@@ -2053,7 +2061,8 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
* We consider only non-mapped and non-allocated blocks
*/
if ((mpd->b_state & (1 << BH_Mapped)) &&
- !(mpd->b_state & (1 << BH_Delay)))
+ !(mpd->b_state & (1 << BH_Delay)) &&
+ !(mpd->b_state & (1 << BH_Unwritten)))
return 0;
/*
* We need to make sure the BH_Delay flag is passed down to
@@ -2205,6 +2214,17 @@ flush_it:
return;
}

+static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
+{
+ /*
+ * unmapped buffer is possible for holes.
+ * delay buffer is possible with delayed allocation.
+ * We also need to consider unwritten buffer as unmapped.
+ */
+ return (!buffer_mapped(bh) || buffer_delay(bh) ||
+ buffer_unwritten(bh)) && buffer_dirty(bh);
+}
+
/*
* __mpage_da_writepage - finds extent of pages and blocks
*
@@ -2289,8 +2309,7 @@ static int __mpage_da_writepage(struct page *page,
* Otherwise we won't make progress
* with the page in ext4_da_writepage
*/
- if (buffer_dirty(bh) &&
- (!buffer_mapped(bh) || buffer_delay(bh))) {
+ if (ext4_bh_unmapped_or_delay(NULL, bh)) {
mpage_add_bh_to_extent(mpd, logical,
bh->b_size,
bh->b_state);
@@ -2318,6 +2337,14 @@ 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
+ *
+ * For delayed buffer_head we have BH_Mapped, BH_New, BH_Delay set.
+ * We also have b_blocknr = -1 and b_bdev initialized properly
+ *
+ * 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)
@@ -2353,28 +2380,23 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
set_buffer_delay(bh_result);
} else if (ret > 0) {
bh_result->b_size = (ret << inode->i_blkbits);
- /*
- * With sub-block writes into unwritten extents
- * we also need to mark the buffer as new so that
- * the unwritten parts of the buffer gets correctly zeroed.
- */
- if (buffer_unwritten(bh_result))
+ if (buffer_unwritten(bh_result)) {
+ /* A delayed write to unwritten bh should
+ * be marked new and mapped. Mapped ensures
+ * that we don't do get_block multiple times
+ * when we write to the same offset and new
+ * ensures that we do proper zero out for
+ * partial write.
+ */
set_buffer_new(bh_result);
+ set_buffer_mapped(bh_result);
+ }
ret = 0;
}

return ret;
}

-static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
-{
- /*
- * unmapped buffer is possible for holes.
- * delay buffer is possible with delayed allocation
- */
- return ((!buffer_mapped(bh) || buffer_delay(bh)) && buffer_dirty(bh));
-}
-
static int ext4_normal_get_block_write(struct inode *inode, sector_t iblock,
struct buffer_head *bh_result, int create)
{
@@ -2828,7 +2850,7 @@ static int ext4_da_should_update_i_disksize(struct page *page,
for (i = 0; i < idx; i++)
bh = bh->b_this_page;

- if (!buffer_mapped(bh) || (buffer_delay(bh)))
+ if (!buffer_mapped(bh) || (buffer_delay(bh)) || buffer_unwritten(bh))
return 0;
return 1;
}
--
1.6.3.2.1.gb9f7d.dirty