Re: [PATCH v2 04/14] mm: add VM_UFFD_RWP VMA flag

[SeongJae Park]

On Fri,  8 May 2026 16:55:16 +0100 "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx> wrote:

> Preparatory patch for userfaultfd read-write protection (RWP). RWP
> extends userfaultfd protection from plain write-protection (WP) to
> full read-write protection: accesses to an RWP-protected range --
> reads as well as writes -- trap through userfaultfd.
> 
> RWP marks ranges by combining PAGE_NONE with the uffd PTE bit, so
> the flag is only meaningful when both primitives exist. A new
> CONFIG_USERFAULTFD_RWP Kconfig symbol auto-selects when CONFIG_64BIT,
> CONFIG_ARCH_HAS_PTE_PROTNONE, and CONFIG_HAVE_ARCH_USERFAULTFD_WP
> are all set; call sites that gate on the flag depend on the symbol.
> Elsewhere VM_UFFD_RWP aliases VM_NONE and every downstream check
> folds to dead code.
> 
> Nothing sets the flag yet.
> 
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Assisted-by: Claude:claude-opus-4-6

A silly but loud thought.  Would it make more sense to put Signed-off-by: after
Assisted-by: ?

> ---
>  Documentation/filesystems/proc.rst |  1 +
>  fs/proc/task_mmu.c                 |  3 +++
>  include/linux/mm.h                 | 28 +++++++++++++++++----------
>  include/linux/userfaultfd_k.h      | 31 +++++++++++++++++++++++++-----
>  include/trace/events/mmflags.h     |  7 +++++++
>  mm/Kconfig                         |  9 +++++++++
>  6 files changed, 64 insertions(+), 15 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index db6167befb7b..db28207c5290 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -607,6 +607,7 @@ encoded manner. The codes are the following:
>      um    userfaultfd missing tracking
>      uw    userfaultfd wr-protect tracking
>      ui    userfaultfd minor fault
> +    ur    userfaultfd read-write-protect tracking

Yet another silly but loud thought.  My first feeling on this was that this
reads like 'u'serfaultfd 'r'ead-protect.  And was further thinking 'uf' for
just 'u'seffaultfd 'f'ault or 'up' for 'u'serfault-'p'rotect might make sense.
But ended up thinking this is too trivial and ain't really matter.

[...]
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e8bf1e9e6ad9..ccf534a8cbc9 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1347,6 +1347,15 @@ config HAVE_ARCH_USERFAULTFD_MINOR
>  	help
>  	  Arch has userfaultfd minor fault support
>  
> +config USERFAULTFD_RWP
> +	def_bool y
> +	depends on 64BIT && ARCH_HAS_PTE_PROTNONE && HAVE_ARCH_USERFAULTFD_WP
> +	help
> +	  Userfaultfd read-write protection (UFFDIO_RWPROTECT) delivers a

Seems UFFDIO_RWPROTECT will be introduced later.  Would it make more sense to
add this config together with the patch?

> +	  userfaultfd notification on every access -- read or write -- to a
> +	  protected range, letting userspace observe the working set of a
> +	  process.
> +
>  menuconfig USERFAULTFD
>  	bool "Enable userfaultfd() system call"
>  	depends on MMU
> -- 
> 2.51.2
> 
> 

None of my comments is a blocker.

Reviewed-by: SeongJae Park <sj@xxxxxxxxxx>


Thanks,
SJ