No, it's not so. If the allocation is backed by the kernel, then it will be zero-filled for security reasons. If it's backed by user-space malloc then who know

No, it's not so. If the allocation is backed by the kernel, then it will be zero-filled for security reasons. If it's backed by user-space malloc then who knows; but there's never a scenario where a mallocated page is quietly replaced by a zero-filled page behind the scenes.