linux-kernel - Re: [PATCH 00/16] expand mmap_prepare functionality, port more users

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Message-ID: <4af2ef49-2695-4880-8d4b-92f11d8f9c7c@lucifer.local>
Date: Mon, 8 Sep 2025 16:15:26 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH 00/16] expand mmap_prepare functionality, port more users

On Mon, Sep 08, 2025 at 12:04:04PM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 03:48:36PM +0100, Lorenzo Stoakes wrote:
> > But sadly some _do need_ to do extra work afterwards, most notably,
> > prepopulation.
>
> I think Jan is suggesting something more like
>
> mmap_op()
> {
>    struct vma_desc desc = {};
>
>    desc.[..] = x
>    desc.[..] = y
>    desc.[..] = z
>    vma = vma_alloc(desc);
>
>    ret = remap_pfn(vma)
>    if (ret) goto err_vma;
>
>    return vma_commit(vma);
>
> err_va:
>   vma_dealloc(vma);
>   return ERR_PTR(ret);
> }
>
> Jason

Right, unfortunately the locking and the subtle issues around memory mapping
really preclude something like this I think. We really do need to keep control
over that.

And since partly the motivation here is 'drivers do insane things when given too
much freedom', I feel this would not improve that :)

If you look at do_mmap() -> mmap_region() -> __mmap_region() etc. you can see a
lot of that.

We also had a security issue arise as a result of incorrect error path handling,
I don't think letting a driver writer handle that is wise.

It's a nice idea, but I just think this stuff is too sensitive for that. And in
any case, it wouldn't likely be tractable to convert legacy code to this.

Cheers, Lorenzo