Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Rik van Riel <riel@redhat.com>
Cc: "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Mike Kravetz" <mike.kravetz@oracle.com>,
	linux-mm <linux-mm@kvack.org>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Colm MacCárthaigh" <colm@allcosts.net>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"Will Drewry" <wad@chromium.org>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Linux API" <linux-api@vger.kernel.org>,
	"Matthew Wilcox" <willy@infradead.org>
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK
Date: Fri, 11 Aug 2017 13:50:07 -0700	[thread overview]
Message-ID: <CA+55aFzXBP-dvVC_q+FMDAxFKE1=PoFX+0FjEnSU+b54VpEKtw@mail.gmail.com> (raw)
In-Reply-To: <1502483265.6577.52.camel@redhat.com>

On Fri, Aug 11, 2017 at 1:27 PM, Rik van Riel <riel@redhat.com> wrote:
>>
>> Yes, you don't do the page table copies. Fine. But you leave vma with
>> the the anon_vma pointer - doesn't that mean that it's still
>> connected
>> to the original anonvma chain, and we might end up swapping something
>> in?
>
> Swapping something in would require there to be a swap entry in
> the page table entries, which we are not copying, so this should
> not be a correctness issue.

Yeah, I thought the rmap code just used the offset from the start to
avoid even doing swap entries, but I guess we don't actually ever
populate the page tables without the swap entry being there.

> There is another test in copy_page_range already which ends up
> skipping the page table copy when it should not be done.

Well, the VM_DONTCOPY test is in dup_mmap(), and I think I'd rather
match up the VM_WIPEONFORK logic with VM_DONTCOPY than with the
copy_page_range() tests.

Because I assume you are talking about the "if it's a shared mapping,
we don't need to copy the page tables and can just do it at page fault
time instead" part? That's a rather different thing, which isn't so
much about semantics, as about just a trade-off about when to touch
the page tables.

But yes, that one *might* make sense in dup_mmap() too. I just don't
think it's really analogous to the WIPEONFORK and DONTCOPY tests.

                      Linus

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