Re: [RFC PATCH 2/6] mm: nommu: use vma_is_anonymous() to check if vmas are anonymous

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Aug 13, 2026 at 03:33:57PM +0900, Hajime Tazaki wrote:
> Private file mappings (like those from /dev/zero) can have vma->vm_file
> set but remain structurally anonymous since they lack vm_ops. Testing
> vma->vm_file instead of vma_is_anonymous(vma) might cause mremap to
> return a spurious -EINVAL when userspace attempts to shrink these
> mappings.
>
> This commit fixes this issue by using vma_is_anonymous() instead of
> testing vma->vm_file to address the case of /dev/zero.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: "Liam R. Howlett" <liam@infradead.org>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Vlastimil Babka <vbabka@kernel.org>
> Cc: Jann Horn <jannh@google.com>
> Cc: Pedro Falcato <pfalcato@suse.de>
> Cc: linux-mm@kvack.org
> Closes: https://sashiko.dev/#/patchset/20260710054648.924005-1-thehajime%40gmail.com
> Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

As staged in review on 1/6 you need to squash this somehow.

> ---
>  mm/nommu.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 89444ee2aca6..e40990e15831 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -1196,7 +1196,7 @@ unsigned long do_mmap(struct file *file,
>  	add_nommu_region(region);
>
>  	/* clear anonymous mappings that don't ask for uninitialized data */
> -	if (!vma->vm_file &&
> +	if (vma_is_anonymous(vma) &&
>  	    (!IS_ENABLED(CONFIG_MMAP_ALLOW_UNINITIALIZED) ||
>  	     !(flags & MAP_UNINITIALIZED)))
>  		memset((void *)region->vm_start, 0,
> @@ -1328,7 +1328,7 @@ static int split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
>
>  	/* we're only permitted to split anonymous regions (these should have
>  	 * only a single usage on the region) */
> -	if (vma->vm_file)
> +	if (!vma_is_anonymous(vma))
>  		return -ENOMEM;
>
>  	mm = vma->vm_mm;
> @@ -1484,7 +1484,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, struct list
>  	}
>
>  	/* we're allowed to split an anonymous VMA but not a file-backed one */
> -	if (vma->vm_file) {
> +	if (!vma_is_anonymous(vma)) {
>  		do {
>  			if (start > vma->vm_start)
>  				return -EINVAL;
> @@ -1617,7 +1617,7 @@ static unsigned long do_mremap(unsigned long addr,
>  		/* like do_munmap(), we're allowed to shrink an anonymous VMA but not
>  		 * a file-backed one
>  		 */
> -		if (vma->vm_file)
> +		if (!vma_is_anonymous(vma))
>  			return (unsigned long) -EINVAL;
>
>  		/* vmi_shrink_vma() needs from/to pointers to be removed,
> --
> 2.43.0
>

--
Cheers, Lorenzo