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Thu, 29 Jan 2026 01:02:44 +0000 Received: from LV3PR12MB9412.namprd12.prod.outlook.com ([fe80::c319:33b5:293:6ec4]) by LV3PR12MB9412.namprd12.prod.outlook.com ([fe80::c319:33b5:293:6ec4%5]) with mapi id 15.20.9542.010; Thu, 29 Jan 2026 01:02:44 +0000 Message-ID: Date: Wed, 28 Jan 2026 17:02:33 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC v6 05/26] nova-core: mm: Add support to use PRAMIN windows to write to VRAM To: Danilo Krummrich , Joel Fernandes Cc: Zhi Wang , linux-kernel@vger.kernel.org, Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Jonathan Corbet , Alex Deucher , Christian Koenig , Jani Nikula , Joonas Lahtinen , Rodrigo Vivi , Tvrtko Ursulin , Huang Rui , Matthew Auld , Matthew Brost , Lucas De Marchi , Thomas Hellstrom , Helge Deller , Alice Ryhl , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , Bjorn Roy Baron , Benno Lossin , Andreas Hindborg , Trevor Gross , Alistair Popple , Timur Tabi , Edwin Peer , Alexandre Courbot , Andrea Righi , Andy Ritger , Alexey Ivanov , Balbir Singh , Philipp Stanner , Elle Rhumsaa , Daniel Almeida , nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, rust-for-linux@vger.kernel.org, linux-doc@vger.kernel.org, amd-gfx@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-fbdev@vger.kernel.org References: <20260120204303.3229303-1-joelagnelf@nvidia.com> <20260120204303.3229303-6-joelagnelf@nvidia.com> <20260121100745.2b5a58e5.zhiw@nvidia.com> Content-Language: en-US From: John Hubbard In-Reply-To: Content-Type: text/plain; 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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