A filesystem’s role is to store information and retrieve it in its original
form on request. But filesystems are also expected to prevent the
retrieval of information by people who should not see it. That requirement
extends to data that has been deleted; users expect that data to be truly
gone and will not welcome its reappearance in surprising places. Some work
being done with ext4 shows the kind of measures that are required to live
up to that expectation.

Source: LWN.net – [$] Preventing information leaks from ext4 filesystems