THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE

On Thu, Mar 03, 2016 at 07:51:50PM +0300, Kirill A. Shutemov wrote:

Truncate and punch hole that only cover part of THP range is implemented
by zero out this part of THP.

This have visible effect on fallocate(FALLOC_FL_PUNCH_HOLE) behaviour.
As we don't really create hole in this case, lseek(SEEK_HOLE) may have
inconsistent results depending what pages happened to be allocated.
Not sure if it should be considered ABI break or not.

Looks like this shouldn't be a problem. man 2 fallocate:

	Within the specified range, partial filesystem blocks are zeroed,
	and whole filesystem blocks are removed from the file.  After a
	successful call, subsequent reads from this range will return
	zeroes.

It means we effectively have 2M filesystem block size.

And I don't see any guarantee about subsequent lseek(SEEK_HOLE) beheviour.

-- 
 Kirill A. Shutemov