linux-kernel - Re: [PATCH RFC v6 05/26] nova-core: mm: Add support to use PRAMIN windows to write to VRAM

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Message-ID: <bd6fcda9-0d76-4208-b6c1-8df6f9f4616e@nvidia.com>
Date: Wed, 28 Jan 2026 17:02:33 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>, Joel Fernandes <joelagnelf@...dia.com>
Cc: Zhi Wang <zhiw@...dia.com>, linux-kernel@...r.kernel.org,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Jonathan Corbet <corbet@....net>, Alex Deucher <alexander.deucher@....com>,
 Christian Koenig <christian.koenig@....com>,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, Tvrtko Ursulin
 <tursulin@...ulin.net>, Huang Rui <ray.huang@....com>,
 Matthew Auld <matthew.auld@...el.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Lucas De Marchi <lucas.demarchi@...el.com>,
 Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
 Helge Deller <deller@....de>, Alice Ryhl <aliceryhl@...gle.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
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 Bjorn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>, Alistair Popple <apopple@...dia.com>,
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Subject: Re: [PATCH RFC v6 05/26] nova-core: mm: Add support to use PRAMIN
 windows to write to VRAM

On 1/28/26 4:09 PM, Danilo Krummrich wrote:
> On Wed Jan 28, 2026 at 4:27 PM CET, Joel Fernandes wrote:
>> I will go over these concerns, just to clarify - do you mean forbidding
>> *any* lock or do you mean only forbidding non-atomic locks? I believe we
>> can avoid non-atomic locks completely - actually I just wrote a patch
>> before I read this email to do just. If we are to forbid any locking at
>> all, that might require some careful redesign to handle the above race
>> afaics.
> 
> It's not about the locks themselves, sleeping locks are fine too. It's about
> holding locks that are held elsewhere when doing memory allocations that can
> call back into MMU notifiers or the shrinker.

If you look at core kernel mm, you'll find a similar constraint: avoid
holding any locks while allocating--unless you are in the reclaim code
itself.

Especially when dealing with page tables.

So this is looking familiar to me and I agree with the constraint, fwiw.

> 
> I.e. if in the fence signalling critical path you wait for a mutex that is held
> elsewhere while allocating memory and the memory allocation calls back into the
> shrinker, you may end up waiting for your own DMA fence to be signaled, which
> causes a deadlock.

Right, and the list of pitfalls such as this is basically limited only
by your imagination--it's long. :)

thanks,
-- 
John Hubbard