linux-kernel - Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX

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Date:   Fri, 17 Feb 2017 12:12:31 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and PR_GET_MAX_VADDR

On Fri, Feb 17, 2017 at 12:02 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Feb 17, 2017 at 6:13 AM, Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
>> This patch introduces two new prctl(2) handles to manage maximum virtual
>> address available to userspace to map.
>
> So this is my least favorite patch of the whole series, for a couple of reasons:
>
>  (a) adding new code, and mixing it with the mindless TASK_SIZE ->
> get_max_addr() conversion.
>
>  (b) what's the point of that whole TASK_SIZE vs get_max_addr() thing?
> When use one, when the other?
>
> so I think this patch needs a lot more thought and/or explanation.
>
> Honestly, (a) is a no-brainer, and can be fixed by just splitting the
> patch up. But I think (b) is more fundamental.
>
> In particular, I think that get_max_addr() thing is badly defined.
> When should you use TASK_SIZE, when should you use TASK_SIZE_MAX, and
> when should you use get_max_addr()? I don't find that clear at all,
> and I think that needs to be a whole lot more explicit and documented.
>
> I also get he feeling that the whole thing is unnecessary. I'm
> wondering if we should just instead say that the whole 47 vs 56-bit
> virtual address is _purely_ about "get_unmapped_area()", and nothing
> else.
>
> IOW, I'm wondering if we can't just say that
>
>  - if the processor and kernel support 56-bit user address space, then
> you can *always* use the whole space
>
>  - but by default, get_unmapped_area() will only return mappings that
> fit in the 47 bit address space.
>
> So if you use MAP_FIXED and give an address in the high range, it will
> just always work, and the MM will always consider the task size to be
> the full address space.

At the very least, I'd want to see
MAP_FIXED_BUT_DONT_BLOODY_UNMAP_ANYTHING.  I *hate* the current
interface.

>
> But for the common case where a process does no use MAP_FIXED, the
> kernel will never give a high address by default, and you have to do
> the process control thing to say "I want those high addresses".
>
> Hmm?

How about MAP_LIMIT where the address passed in is interpreted as an
upper bound instead of a fixed address?

--Andy