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>
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 @@
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 @@
* 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;