tree 6fa4fded382ea053ec6ef1d87ec83013a8037e11
parent 5b61de757535095c99212c1ed857c3a0e0bbe386
author Theodore Ts'o <tytso@mit.edu> 1376705213 -0400
committer Theodore Ts'o <tytso@mit.edu> 1376705213 -0400

ext4: allocate delayed allocation blocks before rename

When ext4_rename() overwrites an already existing file, call
ext4_alloc_da_blocks() before starting the journal handle which
actually does the rename, instead of doing this afterwards.  This
improves the likelihood that the contents will survive a crash if an
application replaces a file using the sequence:

1)  write replacement contents to foo.new
2)  <omit fsync of foo.new>
3)  rename foo.new to foo

It is still not a guarantee, since ext4_alloc_da_blocks() is *not*
doing a file integrity sync; this means if foo.new is a very large
file, it may not be completely flushed out to disk.

However, for files smaller than a megabyte or so, any dirty pages
should be flushed out before we do the rename operation, and so at the
next journal commit, the CACHE FLUSH command will make sure al of
these pages are safely on the disk platter.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>