Re: [PATCH] ext4: Add support for SFITRIM, an ioctl for secure FITRIM.

[Dave Chinner <david@fromorbit.com>]

On Thu, Jun 12, 2014 at 09:37:58PM -0700, JP Abgrall wrote:
> On Thu, Jun 12, 2014 at 8:30 PM, Dave Chinner <david@fromorbit.com> wrote:
> > On Fri, Jun 13, 2014 at 01:15:38PM +1000, Dave Chinner wrote:
> >> Indeed, mixing -o discard and SFITRIM is a recipe for
> >> confusion and leakage - "but I used secure trim on the device!" -
> >> and so all discards either have to be secure or not.
> 
> The idea was to keep on not using -o discard. And move from FITRIM to SFITRIM.

IOWs, you want either normal discard or secure discard, and not a
mix of both. IOWs, you don't need SFITRIM, you need a "block device
does secure discard only" configuration flag...

> > Oh, and while I think of it secure discard at the filesystem level
> > isn't even a guarantee that you'll get rid of all stale references
> > to a sector - if the filesystem has freed and then re-allocated a
> > block without having gone through a discard cycle on that block,
> > then the underlying device may have old copies of the block that it
> > hasn't garbage collected and SFITRIM won't clean those up because it
> > won't ask to trim in-use blocks....
> 
> Arg. So, if understand this correctly, if the eMMC chip won't get a
> secure discard/trim of a block that gets reassigned to the FS, then
						      ^^ within

> data duplicates within the eMMC related to that block are not cleared,
> and the next SFITRIM won't even reach that block or the duplicates as
> the FS says they are in use.

Pretty much.

And even using -o discard is no guarantee that the filesystem will
issue a discard between freeing and re-using a block e.g.  XFS
explicitly avoids issuing discards for blocks it re-uses immediately
because they are always considered "in-use" from a transactional
POV. Hence there is no place where the block is considered free, and
hence there isn't a point in time where a discard can be safely
issued on that block.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com