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

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Message-ID: <182bfb4f-e856-47d8-bc54-d419109cb4ac@cachyos.org>
Date: Tue, 8 Apr 2025 15:52:25 +0800
From: Eric Naim <dnaim@...hyos.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Christoph Hellwig <hch@...radead.org>
Cc: 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 4/8/25 14:25, Lorenzo Stoakes wrote:
> 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.


This seems to be the case, upon looking a bit deeper it looks like the driver code 
is calling atleast one of vm_flags_set. I couldn't find any direct calls to {,__}vma_start_write
at first and was bit confused.

> 
> 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 :)

Yeah, as obvious as it seems it doesn't happen on their open-sourced code :)

Either way, I'm thankful for the replies. NVIDIA *should* have probably noticed
this already and it would probably fixed in the next driver or two so I'll just
let sleeping dogs lie.


-- 
Regards,
Eric