From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9DED04BAA09; Mon, 31 Aug 2026 14:03:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788185028; cv=none; b=OAS0s8LaPhpM7xolubKo1F+VysBKURwEs3+giSQ403hZwsYfbGhcHk4AhHcl8GG5z4OUa6F2/HzwBFycAoWDz0SnCnpQOZNOLid0gn3X6YDkV5ydukvZg0Wlfyvv30brWlz5AlM+ixcjvLl8o1uo6yK9hirHsSuwgC3ZS0g3ZpI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788185028; c=relaxed/simple; bh=yNTKZ1gEh2b/vXha+oKOsZE/GcNfhLewQ7JdBaCKgjU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MErFZW1tFvS+jRl+CdVgrp6fuSlj9myg91DlyOHUzh/gRDA0eoWH772BbONZXlqQB/rJw5Hu70/JdV+elaRScAIuoCuu7TUry7qU5jjPC+z+5aVjLPDNJsb53V/CK95GCl/T36ZTjquu9TJAYT0Cxn9Gp+RXiyj8F2pX+xhNW9w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=DC63ksHd; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="DC63ksHd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 27C681F00A3D; Mon, 31 Aug 2026 14:03:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788185027; bh=1HX9TYQjgO/5hXmQFqlxHA27MLAvo/v6Sb6ISr7PR1Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=DC63ksHdTNXpk1d7LsdZXGkJ4TqoesWNQd4cA0cdBSN2sTz5oarOWquXAAe7Xne88 x2+W0buRDqFXeuIdOYmT/SKvqfrbNwFHWN7ihOHDbZt+bSCqz67ja1m8+d/tMnrnTj mIlGeZbmnVY1BH9bhtNYamzgigzi5iLBVI+JBqh4oCQQg783Gs7v5Iw7Ym0936oesb BoOyDnrhCglt26iK1OImhRBZEsl1owvpJVF67euhl7waxIXyn09LKFp9Uc91hAJVmA eZaBpydeKgXBFIkT7tX1ms8GB0huF8Q0Crk4ypZoD5NpaNrSRbRfh3C4iKZXcAZD/r sWBaijMSt89kg== Message-ID: <0849e5ca-6dec-4cf9-83ac-89ab08364044@kernel.org> Date: Mon, 31 Aug 2026 16:03:38 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 06/10] Reclaim memory from blocked kernel stacks Content-Language: en-US To: Sebastian Andrzej Siewior , Matthew Wilcox Cc: Peter Zijlstra , David Stevens , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H . Peter Anvin" , Andrew Morton , Dave Chinner , Qi Zheng , Roman Gushchin , Muchun Song , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak , Uladzislau Rezki , David Hildenbrand , Lorenzo Stoakes , "Liam R . Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Clark Williams , suleiman@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-rt-devel@lists.linux.dev References: <20260827232948.2520558-1-stevensd@google.com> <20260827232948.2520558-7-stevensd@google.com> <20260828133620._x2XfJR_@linutronix.de> <20260828135947.GU776954@noisy.programming.kicks-ass.net> <20260829084901.GA776954@noisy.programming.kicks-ass.net> <20260831073619.Ssq-wt7s@linutronix.de> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20260831073619.Ssq-wt7s@linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/31/26 09:36, Sebastian Andrzej Siewior wrote: > On 2026-08-29 15:49:22 [+0100], Matthew Wilcox wrote: >> On Sat, Aug 29, 2026 at 10:49:01AM +0200, Peter Zijlstra wrote: >> > > On PREEMPT_RT, it is necessary to skip the call to >> > > alloc_pages_nolock_noprof() from under pi_lock, since with that >> > > configuration spin_trylock can end up needing to take pi_lock. But at >> > > least on !PREEMPT_RT, there is no risk of deadlock. >> > >> > Yeah, which puts the lie to that horrific hack Alexei did. We should >> > probably teach lockdep about spin_trylock() not being safe and see the >> > house of cards crumble. Is it already happening then? https://lore.kernel.org/all/20260831053846.107974-1-ngocthang2710.1999@gmail.com/ >> Why is spin_trylock() unsafe on PREEMPT_RT? It's not intuitive since >> one can mutex_trylock() in interrupt context or under spinlock. > > no, you shouldn't do mutex_trylock() in IRQ. There was something, I > don't remember but that is the reason people use semaphores because it > is okay to do down_trylock() in IRQ. It might be unlock path. Is it what this comment (recently moved to can_spin_trylock()) explains? /* * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is * unsafe in NMI. If spin_trylock() is called from hard IRQ the current * task may be waiting for one rt_spin_lock, but rt_spin_trylock() will * mark the task as the owner of another rt_spin_lock which will * confuse PI logic, so return immediately if called from hard IRQ or * NMI. * * Note, irqs_disabled() case is ok. spin_trylock() can be called * from raw_spin_lock_irqsave region. */ if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq())) return false; Of course checking for being under raw_spinlock_t can't be added like this, hm. > Sebastian