linux-kernel - Re: [PATCH 2/2] mm,fork: introduce MADV

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Date:   Fri, 11 Aug 2017 13:50:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-mm <linux-mm@...ck.org>,
        Florian Weimer <fweimer@...hat.com>,
        Colm MacCárthaigh <colm@...costs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux API <linux-api@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 2/2] mm,fork: introduce MADV_WIPEONFORK

On Fri, Aug 11, 2017 at 1:27 PM, Rik van Riel <riel@...hat.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