Re: THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE

On 03/04/2016 03:26 AM, Kirill A. Shutemov wrote:

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

quoted

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.

The question is still whether this will case problems for apps.

Isn't 2MB a quote unusual block size?  Wouldn't some files on a tmpfs
filesystem act like they have a 2M blocksize and others like they have
4k?  Would that confuse apps?

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