Re: [PATCH] mm: make MEMFD_CREATE into a selectable config option

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jun 30, 2023 at 11:08:53AM +0200, Thomas Weißschuh wrote:
> The memfd_create() syscall, enabled by CONFIG_MEMFD_CREATE, is useful on
> its own even when not required by CONFIG_TMPFS or CONFIG_HUGETLBFS.

If you don't have tmpfs or hugetlbfs enabled, then what fs ends up
backing the file returned by memfd_create()?  ramfs?

(Not an objection, I'm just curious...)

--D

> Split it into its own proper bool option that can be enabled by users.
> 
> Move that option into mm/ where the code itself also lies.
> Also add "select" statements to CONFIG_TMPFS and CONFIG_HUGETLBFS so
> they automatically enable CONFIG_MEMFD_CREATE as before.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  fs/Kconfig | 5 ++---
>  mm/Kconfig | 3 +++
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 18d034ec7953..19975b104bc3 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -169,6 +169,7 @@ source "fs/sysfs/Kconfig"
>  config TMPFS
>  	bool "Tmpfs virtual memory file system support (former shm fs)"
>  	depends on SHMEM
> +	select MEMFD_CREATE
>  	help
>  	  Tmpfs is a file system which keeps all files in virtual memory.
>  
> @@ -240,6 +241,7 @@ config HUGETLBFS
>  	bool "HugeTLB file system support"
>  	depends on X86 || IA64 || SPARC64 || ARCH_SUPPORTS_HUGETLBFS || BROKEN
>  	depends on (SYSFS || SYSCTL)
> +	select MEMFD_CREATE
>  	help
>  	  hugetlbfs is a filesystem backing for HugeTLB pages, based on
>  	  ramfs. For architectures that support it, say Y here and read
> @@ -264,9 +266,6 @@ config HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON
>  	  enable HVO by default. It can be disabled via hugetlb_free_vmemmap=off
>  	  (boot command line) or hugetlb_optimize_vmemmap (sysctl).
>  
> -config MEMFD_CREATE
> -	def_bool TMPFS || HUGETLBFS
> -
>  config ARCH_HAS_GIGANTIC_PAGE
>  	bool
>  
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 09130434e30d..22acffd9009d 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1144,6 +1144,9 @@ config KMAP_LOCAL_NON_LINEAR_PTE_ARRAY
>  config IO_MAPPING
>  	bool
>  
> +config MEMFD_CREATE
> +	bool "Enable memfd_create() system call" if EXPERT
> +
>  config SECRETMEM
>  	default y
>  	bool "Enable memfd_secret() system call" if EXPERT
> 
> ---
> base-commit: e55e5df193d247a38a5e1ac65a5316a0adcc22fa
> change-id: 20230629-config-memfd-be6af03b7dca
> 
> Best regards,
> -- 
> Thomas Weißschuh <linux@weissschuh.net>
>