linux-kernel - Re: [PATCH v2 11/16] mm: update mem char driver to use mmap

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Message-ID: <20250918191119.3622358-1-clm@meta.com>
Date: Thu, 18 Sep 2025 12:11:05 -0700
From: Chris Mason <clm@...a.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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Subject: Re: [PATCH v2 11/16] mm: update mem char driver to use mmap_prepare

On Wed, 10 Sep 2025 21:22:06 +0100 Lorenzo Stoakes <lorenzo.stoakes@...cle.com> wrote:

> Update the mem char driver (backing /dev/mem and /dev/zero) to use
> f_op->mmap_prepare hook rather than the deprecated f_op->mmap.
> 
> The /dev/zero implementation has a very unique and rather concerning
> characteristic in that it converts MAP_PRIVATE mmap() mappings anonymous
> when they are, in fact, not.
> 
> The new f_op->mmap_prepare() can support this, but rather than introducing
> a helper function to perform this hack (and risk introducing other users),
> simply set desc->vm_op to NULL here and add a comment describing what's
> going on.
> 
> We also introduce shmem_zero_setup_desc() to allow for the shared mapping
> case via an f_op->mmap_prepare() hook, and generalise the code between this
> and shmem_zero_setup().
> 
> We also use the desc->action_error_hook to filter the remap error to
> -EAGAIN to keep behaviour consistent.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>  drivers/char/mem.c       | 75 ++++++++++++++++++++++------------------
>  include/linux/shmem_fs.h |  3 +-
>  mm/shmem.c               | 40 ++++++++++++++++-----
>  3 files changed, 76 insertions(+), 42 deletions(-)
> 

[ ... ]

> diff --git a/mm/shmem.c b/mm/shmem.c
> index 990e33c6a776..cb6ff00eb4cb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c

[ ... ]

> @@ -5920,6 +5925,25 @@ int shmem_zero_setup(struct vm_area_struct *vma)
>  	return 0;
>  }
>  
> +/**
> + * shmem_zero_setup_desc - same as shmem_zero_setup, but determined by VMA
> + * descriptor for convenience.
> + * @desc: Describes VMA
> + * Returns: 0 on success, or error
> + */
> +int shmem_zero_setup_desc(struct vm_area_desc *desc)
> +{
> +	struct file *file = __shmem_zero_setup(desc->start, desc->end, desc->vm_flags);
> +
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +
> +	desc->vm_file = file;
> +	desc->vm_ops = &shmem_anon_vm_ops;
> +
> +	return 0;
> +}
> +

Hi Lorenzo,

shmem_zero_setup() does a if (vma->vm_file) fput(vma->vm_file) dance.

It looks like we need one here too?

-chris