On 07/12/2012 07:01 AM, Dave Chinner wrote:

On Wed, Jul 11, 2012 at 11:55:34AM -0700, Hugh Dickins wrote:

quoted

On Wed, 11 Jul 2012, Cong Wang wrote:

quoted

On Mon, 09 Jul 2012 at 22:41 GMT, Hugh Dickins [off-list ref] wrote:

quoted

Revert 4fb5ef089b28 ("tmpfs: support SEEK_DATA and SEEK_HOLE").
I believe it's correct, and it's been nice to have from rc1 to rc6;
but as the original commit said:

I don't know who actually uses SEEK_DATA or SEEK_HOLE, and whether it
would be of any use to them on tmpfs.  This code adds 92 lines and 752
bytes on x86_64 - is that bloat or worthwhile?

I don't think 752 bytes matter much, especially for x86_64.

quoted

Nobody asked for it, so I conclude that it's bloat: let's revert tmpfs
to the dumb generic support for v3.5.  We can always reinstate it later
if useful, and anyone needing it in a hurry can just get it out of git.

If you don't have burden to maintain it, I'd prefer to leave as it is,
I don't think 752-bytes is the reason we revert it.

Thank you, your vote has been counted ;)
and I'll be glad if yours stimulates some agreement or disagreement.

But your vote would count for a lot more if you know of some app which
would really benefit from this functionality in tmpfs: I've heard of none.

So what? I've heard of no apps that use this functionality on XFS,
either, but I have heard of a lot of people asking for it to be
implemented over the past couple of years so they can use it.
There's been patches written to make coreutils (cp) make use of it
instead of parsing FIEMAP output to find holes, though I don't know
if that's gone beyond more than "here's some patches"...

Yes, for apps, cp(1) will make use of it to replace the old FIEMAP for efficient sparse file copy.
I have implemented an extent-scan module to coreutils a few years ago,
http://fossies.org/dox/coreutils-8.17/extent-scan_8c_source.html

It does extent scan through FIEMAP, however, SEEK_DATA/SEEK_HOLE is more convenient and easy to use
considering the call interface.  So FIEMAP will be replaced by SEEK_XXX once it got supported by EXT4.

Moreover, I have discussed with Jim who is the coreutils maintainer previously, He would like to post
extent-scan module to Gnulib so that other GNU utilities which are relied on Gnulib might be a potential
user of it, at least, GNU tar will definitely need it for sparse file backup.

Besides, given that you can punch holes in tmpfs files, it seems
strange to then say "we don't need a method of skipping holes to
find data quickly"....

So its deserve to keep this feature working on tmpfs considering hole punch. :)

Thanks,
-Jeff

Besides, seek-hole/data is still shiny new and lots of developers
aren't even aware of it's presence in recent kernels. Removing new
functionality saying "no-one is using it" is like smashing the egg
before the chicken hatches (or is it cutting of the chickes's head
before it lays the egg?).

Cheers,

Dave.

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