On Tue, 17 Jun 2014, Dave Chinner wrote:

Date: Tue, 17 Jun 2014 12:49:53 +1000
From: Dave Chinner <david@fromorbit.com>
To: Theodore Ts'o <tytso@mit.edu>
Cc: JP Abgrall <redacted>, Eric Sandeen <redacted>,
    linux-ext4@vger.kernel.org, Geremy Condra [off-list ref],
    "linux-fsdevel@vger.kernel.org" [off-list ref]
Subject: Re: [PATCH] ext4: Add support for SFITRIM,
    an ioctl for secure FITRIM.

On Fri, Jun 13, 2014 at 07:41:34PM -0400, Theodore Ts'o wrote:

quoted

On Fri, Jun 13, 2014 at 12:44:34PM -0700, JP Abgrall wrote:

quoted

The per-file secure discard seems to be the way to go, as there are
only a few places in Android where this needs to be turned on.
The  current idletime-fstrim would  switch from FITRIM to SFITRIM to
reduce the leftovers.

OK, how about this?  The following patch is in the Google data center
kernel, but I never got around to get it upstream (oops, was on my
todo list, but it never happened).

If you want to adopt this for usptream, and add support for
BLKSECDISCARD as well as BLKDISCARD, then you could for each file that
you want to do the per-file secure discard, you would just have to
open the file, call the BLKSECDISCARD ioctl, and then delete the file.

Cheers,

					- Ted

commit 16ff6352b123aa134417793d636f05cd4e240eaa
Author: Theodore Ts'o [off-list ref]
Date:   Fri Dec 20 12:48:26 2013 -0500

    ext4: add support for the BLKDISCARD ioctl
    
    The blkdicard ioctl previously only worked on block devices.  Allow
    this ioctl to work on ext4 files.
    
    This commit is intended to be sent upstream.

Not in that form - it's an ugly API hack.

This is really just an extension of hole punching (if the blocks in
the file are being removed) or zeroing (if the blocks are being
retained by the file). Either way, fallocate() is the interface
used for per-file block level manipulations, and either of these
operations could issue a discard (secure or not) during the
punch/zero operation....

I definitely agree with Dave here it is an ugly API hack. Fallocate
seems much more suitable for this.

New flag FALLOC_FL_ISSUE_DISCARD which would work with
FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE and possibly
FALLOC_FL_COLLAPSE_RANGE might actually be useful.

-Lukas

Cheers,

Dave.