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Date:   Wed, 24 Apr 2019 13:20:06 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Matthew Garrett <mjg59@google.com>
Cc:     linux-mm@kvack.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: Allow userland to request that the kernel clear
 memory on release
Message-ID: <20190424202006.GH19031@bombadil.infradead.org>
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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@infradead.org> 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.