SEEK_HOLE defined but useless on linux-3.4+/ext4 [Re: small ascii files can be sparse

[Jim Meyering]

Paul Eggert wrote:
> On further thought, the heuristic is also incorrect for file
> systems that compress their data.  So I installed this further
> patch.
>
> Oh, well.  At least the code is simpler now.  Simple and slow
> is better than complicated and fast and occasionally wrong.
...
> Subject: [PATCH] grep: don't falsely report compressed text files as binary
>
> * NEWS: Document this.
> * src/main.c (file_is_binary): Remove the heuristic based on
> st_blocks, as it does not work for compressed file systems.
> On Solaris, it'd be cheap to test whether the file system is known
> to be uncompressed, which allow the heuristic, but Solaris has
> SEEK_HOLE so there's little point.

Hi Paul,

Without the st_blocks-based heuristic, grep's big-hole test now fails
(exhausts memory and exits with status 2) on an ext4 file system with
a recent linux kernel.
That happens because while SEEK_HOLE and SEEK_DATA are now defined,
the kernel's ext4 lseek/SEEK_HOLE support is just a stub that simply
returns the length of the file.

For the record, the SEEK_HOLE support for btrfs and xfs in
linux-3.4.4 (F17) works the way I would expect, and it looks
like ocfs2 is fine, too.

Here's a demo:

SEEK_HOLE works (detects the hole) with btrfs (SEEK_HOLE == 4):

    $ perl -e '$f=*STDERR; sysseek($f,2**22,0); syswrite($f,"a");' \
      -e 'print 0+sysseek($f,0,4)' 2> j; stat -f --fo=\ %T .
    0 btrfs

SEEK_HOLE is not usable (reports "hole" at EOF) with ext4:
stat -f report ext2/ext3, but that's only looking at the magic number.
It's really ext4:

    $ perl -e '$f=*STDERR; sysseek($f,2**22,0); syswrite($f,"a");' \
      -e 'print 0+sysseek($f,0,4)' 2> j; stat -f --fo=\ %T .
    4194305 ext2/ext3

tmpfs uses the same code,

    4194305 tmpfs