Re: [PATCH] mm: Allow userland to request that the kernel clear memory on release

[Matthew Wilcox]

On Wed, Apr 24, 2019 at 12:33:11PM -0700, Matthew Garrett wrote:
> On Wed, Apr 24, 2019 at 12:28 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > But you can't have a new PageFlag.  Can you instead zero the memory in
> > unmap_single_vma() where we call uprobe_munmap() and untrack_pfn() today?
> 
> Is there any way the page could be referenced by something other than
> a VMA at this point? If so we probably don't want to zero it here, but
> we do want to zero it when the page is finally released (which is why
> I went with a page flag)

It could be the target/source of direct I/O, or userspace could have
registered it with an RDMA device, or ...

It depends on the semantics you want.  There's no legacy code to
worry about here.  I was seeing this as the equivalent of an atexit()
handler; userspace is saying "When this page is unmapped, zero it".
So it doesn't matter that somebody else might be able to reference it --
userspace could have zeroed it themselves.