linux-kernel - Re: [PULL] fixes around VM_PFNMAP and follow

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Date:   Mon, 22 Feb 2021 17:42:38 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux MM <linux-mm@...ck.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>
Subject: Re: [PULL] fixes around VM_PFNMAP and follow_pfn for 5.12 merge window

On Mon, Feb 22, 2021 at 2:25 AM Daniel Vetter <daniel@...ll.ch> wrote:
>
> Cc all the mailing lists ... my usual script crashed and I had to
> hand-roll the email and screwed it up ofc :-/

Oh, and my reply thus also became just a reply to you personally.

So repeating it here, in case somebody has comments about that
access_process_vm() issue.

On Mon, Feb 22, 2021 at 2:23 AM Daniel Vetter <daniel@...ll.ch> wrote:
>
> I've stumbled over this for my own learning and then realized there's a
> bunch of races around VM_PFNMAP mappings vs follow pfn.
>
> If you're happy with this [..]

Happy? No. But it seems an improvement.

I did react to some of this: commit 0fb1b1ed7dd9 ("/dev/mem: Only set
filp->f_mapping") talks about _what_ it does, but not so much _why_ it
does it. It doesn't seem to actually matter, and seems almost
incidental (because you've looked at f_mapping and i_mapping just
didn't matter but was adjacent.

And generic_access_phys() remains horrific. Does anything actually use
this outside of the odd magical access_remote_vm() code?

I'm wondering if that code shouldn't just be removed entirely. It's
quite old, I'm not sure it's really relevant. See commit 28b2ee20c7cb
("access_process_vm device memory infrastructure").

I guess you do debug the X server, but still.. Do you actually ever
look at device memory through the debugger? I'd hope that you'd use an
access function and make gdb call it in the context of the debuggee?

Whatever. I've pulled it, and I'm not _unhappy_ with it, but I'd also
not call myself overly giddy and over the moon happy about this code.

             Linus