populate: punch files after writing to fragment free space properly

populate: punch files after writing to fragment free space properly

The filesystem population code frequently allocates a large file and
punches out every other block ("swiss-cheese files") in an attempt to
cause the creation of a lot of metadata to fill out the btrees.  This
pattern, however, has a subtle bug if the writes to the swiss-cheese
file are not allocated in batches and we're trying to fragment the free
space records in order to achieve a certain metadata btree shape.

This is exactly what happens on a DAX filesystem, since we no longer
have the page cache to stage delalloc writes.  Each xfs_io pwrite call
to the multi-megabyte swiss-chese file turns into multiple 4k pwrites,
which means that file data blocks are allocated 4k at a time.  This can
be fatal to our goal of fragmenting the free space btrees because the
allocator sees a 4k allocation request and uses 4k blocks from the
fragmented parts of the free space to satisfy the "small" request.  When
this happens, the XFS populate function cannot fill out the free space
btree to sufficient height and tests fail.

(In regular delalloc mode we'd cache all those small write() in memory
and try for a single large allocation, which we'd generally get.)

To fix this, we need to force the filesystem to allocate all blocks
before freeing any blocks.  Split the creation of swiss-cheese files
into two parts: (a) writing data to the file to force allocation, and
(b) punching the holes to fragment free space.

This bug affects only XFS but we convert the one ext4 usage anyway.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>