linux-kernel - Re: [PATCH v3 11/13] mm: update mem char driver to use mmap

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Message-ID: <cc8e295d-83ec-4172-a541-21b877349a73@lucifer.local>
Date: Tue, 16 Sep 2025 19:02:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH v3 11/13] mm: update mem char driver to use mmap_prepare

On Tue, Sep 16, 2025 at 02:40:06PM -0300, Jason Gunthorpe wrote:
> 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.

Yeah I don't love it, people get antsy sometimes about changing what the
error code is.

I'd rather pass through than filter but there we are.

>
> But oh well
>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Thanks!

>
> > 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?

Ack! Let me look into whether vma.c caller can use that also.

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

The arch code sure can be fun!

>
> But the code looks fine

Cheers!

>
> Jason

Cheers, Lorenzo