mm for fs: add truncate_pagecache_range()

mm for fs: add truncate_pagecache_range()

Holepunching filesystems ext4 and xfs are using truncate_inode_pages_range
but forgetting to unmap pages first (ocfs2 remembers).  This is not really
a bug, since races already require truncate_inode_page() to handle that
case once the page is locked; but it can be very inefficient if the file
being punched happens to be mapped into many vmas.

Provide a drop-in replacement truncate_pagecache_range() which does the
unmapping pass first, handling the awkward mismatch between arguments to
truncate_inode_pages_range() and arguments to unmap_mapping_range().

Note that holepunching does not unmap privately COWed pages in the range:
POSIX requires that we do so when truncating, but it's hard to justify,
difficult to implement without an i_size cutoff, and no filesystem is
attempting to implement it.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Ben Myers <[email protected]>
Cc: Alex Elder <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>