linux-kernel - Re: [PATCH v7 10/17] mm: uninline the main body of vma_start

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Message-ID: <3f9f8a06-a044-4bce-a4e6-f17090cb3c0f@lucifer.local>
Date: Tue, 8 Apr 2025 07:25:13 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Eric Naim <dnaim@...hyos.org>, Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        peterz@...radead.org, willy@...radead.org, liam.howlett@...cle.com,
        mhocko@...e.com, hannes@...xchg.org, mjguzik@...il.com,
        oliver.sang@...el.com, mgorman@...hsingularity.net, david@...hat.com,
        peterx@...hat.com, oleg@...hat.com, dave@...olabs.net,
        paulmck@...nel.org, brauner@...nel.org, dhowells@...hat.com,
        hdanton@...a.com, hughd@...gle.com, lokeshgidra@...gle.com,
        minchan@...gle.com, jannh@...gle.com, shakeel.butt@...ux.dev,
        souravpanda@...gle.com, pasha.tatashin@...een.com,
        klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 10/17] mm: uninline the main body of vma_start_write()

On Mon, Apr 07, 2025 at 11:01:46PM -0700, Christoph Hellwig wrote:
> On Tue, Apr 08, 2025 at 12:39:25PM +0800, Eric Naim wrote:
> > The out-of-tree NVIDIA modules seem to rely on this symbol, is it possible to use EXPORT_SYMBOL() here instead of EXPORT_SYMBOL_GPL(), below is the modpost error:
>
> No.  They don't have any business using this.

What on _earth_ are they using this for? Is this just via the VMA flag
manipulation functions? If it's something else, it's an unintended use of this.

Anyway, generally speaking - agreed, this is absolutely a no-go Eric. In my view
we simply should not be using EXPORT_SYMBOL() for _any_ new symbols whatsoever.

Out-of-tree modules are simply a non-consideration for core mm code, this is a
GPL open source project. If I had my way we'd simply revoke _all_
EXPORT_SYMBOL()'s, not add new ones.

>
> In fact vma_start_write should not be exported at all, just the
> vm_flags_{set,clear,mod} helpers.

Yup, I'd rather we just kept vma_start_write() mm-internal, though of course
kernel/fork.c (ugh) needs it (we could probably refactor that in some way to
avoid), and literally just the PPC arch (again maybe we can find a way round
that).

Maybe one for me to look at actually... hmm.

Anyway Eric - I wonder if this is simply the nvidia OOT driver doing a
vm_flags_...() call and then having an issue because the lock is uninlined now?

I guess you are jut noticing this is breaking and don't know since - proprietary
code.

Anyway in this case, the OOT driver should just write some GPL wrapper code or
something here. Or better yet - make the driver open source :)