ext4: Mark the unwritten buffer_head as mapped during write_begin

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>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index a953214f2829..ea5c47608cea 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;