linux-kernel - Re: [PATCH] /proc/pid/smaps: add mo info for vma in NOMMU system

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Message-ID: <e4d9e46d-37c7-478d-b543-07ac91413e5b@redhat.com>
Date: Sat, 7 Jun 2025 19:58:58 +0200
From: David Hildenbrand <david@...hat.com>
To: wangfushuai <wangfushuai@...du.com>, akpm@...ux-foundation.org,
 andrii@...nel.org, osalvador@...e.de, Liam.Howlett@...cle.com,
 christophe.leroy@...roup.eu
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] /proc/pid/smaps: add mo info for vma in NOMMU system

On 07.06.25 18:53, wangfushuai wrote:
> Add mo in /proc/[pid]/smaps to indicate vma is marked VM_MAYOVERLAY,
> which means the file mapping may overlay in NOMMU system.
> 
> Fixes: b6b7a8faf05c ("mm/nommu: don't use VM_MAYSHARE for MAP_PRIVATE mappings")
> Signed-off-by: wangfushuai <wangfushuai@...du.com>
> ---
>   Documentation/filesystems/proc.rst | 1 +
>   fs/proc/task_mmu.c                 | 4 ++++
>   2 files changed, 5 insertions(+)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 2a17865dfe39..d280594656a3 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -609,6 +609,7 @@ encoded manner. The codes are the following:
>       uw    userfaultfd wr-protect tracking
>       ss    shadow/guarded control stack page
>       sl    sealed
> +    mo    may overlay file mapping
>       ==    =======================================
>   
>   Note that there is no guarantee that every flag and associated mnemonic will
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 27972c0749e7..ad08807847de 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -970,7 +970,11 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
>   		[ilog2(VM_HUGEPAGE)]	= "hg",
>   		[ilog2(VM_NOHUGEPAGE)]	= "nh",
>   		[ilog2(VM_MERGEABLE)]	= "mg",
> +#ifdef CONFIG_MMU
>   		[ilog2(VM_UFFD_MISSING)]= "um",
> +#else
> +		[ilog2(VM_MAYOVERLAY)]	= "mo",
> +#endif
>   		[ilog2(VM_UFFD_WP)]	= "uw",
>   #ifdef CONFIG_ARM64_MTE
>   		[ilog2(VM_MTE)]		= "mt",

That is mostly an internal-only flag. Why would we want to expose that 
to user space?

-- 
Cheers,

David / dhildenb