linux-kernel - Re: [PATCH v8 22/23] mm: Enable PTE markers by default

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Message-ID: <Yl7RrKV5mXtNAAzi@cmpxchg.org>
Date:   Tue, 19 Apr 2022 11:13:48 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH v8 22/23] mm: Enable PTE markers by default

Hi Peter,

On Mon, Apr 04, 2022 at 09:49:29PM -0400, Peter Xu wrote:
> Enable PTE markers by default.  On x86_64 it means it'll auto-enable
> PTE_MARKER_UFFD_WP as well.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/Kconfig | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 6e7c2d59fa96..3eca34c864c5 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -911,12 +911,14 @@ config ANON_VMA_NAME
>  
>  config PTE_MARKER
>  	bool "Marker PTEs support"
> +	default y
>  
>  	help
>  	  Allows to create marker PTEs for file-backed memory.

make oldconfig just prompted me on these:

---
Marker PTEs support (PTE_MARKER) [Y/n/?] (NEW) ?

CONFIG_PTE_MARKER:

Allows to create marker PTEs for file-backed memory.

Symbol: PTE_MARKER [=y]
Type  : bool
Defined at mm/Kconfig:1046
  Prompt: Marker PTEs support
  Location:
    Main menu
      -> Memory Management options
---

>  config PTE_MARKER_UFFD_WP
>  	bool "Marker PTEs support for userfaultfd write protection"
> +	default y
>  	depends on PTE_MARKER && HAVE_ARCH_USERFAULTFD_WP

It's not possible to answer them without looking at the code.

But after looking at the code, I'm still not sure why it asks
me. Isn't this infrastructure code?

Wouldn't it make more sense to remove the prompt string and have
userfaultfd simply select those?

If this is too experimental to enable per default, a more reasonable
question for the user would be a "userfaultfd file support" option or
something, and have *that* select the marker code.

Thanks!