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: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
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