Re: [PATCH v3 11/13] mm: update mem char driver to use mmap_prepare

Jason Gunthorpe
2025-09-16T17:40:11Z

On Tue, Sep 16, 2025 at 03:11:57PM +0100, Lorenzo Stoakes 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.

Hurm, in practice this converts reserve_pfn_range()/etc conflicts into
from EINVAL into EAGAIN and converts all the unlikely OOM ENOMEM
failures to EAGAIN. Seems wrong/unnecessary to me, I wouldn't have
preserved it.

But oh well

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 0e47465ef0fd..5b368f9549d6 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h

This little bit should probably be its own patch "Add
shmem_zero_setup_desc()", and I wonder if the caller from vma.c can
call the desc version now?

Too bad the usage in ppc is such an adventure through sysfs :\

But the code looks fine

Jason