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 0A71433B6F1; Tue, 28 Jul 2026 15:54:53 +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=1785254095; cv=none; b=IXUr/2Rlf0wvelcBzG70bEbFKuTFSp0z+Gs0Mrmvz8S4h7rQNWFBCI90XWtlLa5IIJZ0HE6FjJqD9PWkOk2GvhzudC2rQNS7ifaN9j9oXWEVmWr0Z5GA8kTOJzolYoUT0h/Erp8UGcfQsI+usqllpBU1W3mwlmrA7Vl5UJbOm0U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785254095; c=relaxed/simple; bh=YyWjg6sPb4sqiwBdGLnmMtTP43HFX17YR5fSxnPEQU4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DPSqgxWQjPQkb/Jf22/gyFlsPM3ypsai8fnzowz9qbZ1DBXWBfiiakhQt593i11bImED72mtcuuBxcMFljdqLdC/OFmd1gS/AkL6G4J9kcDNVHzJ/oVWvVWcfD6nEM2ex/DvGGd0N8FsNTJlUNbcwdjRuQ++PdAI/hoHm439xGQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=S6fTps3w; 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="S6fTps3w" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 788641F000E9; Tue, 28 Jul 2026 15:54:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785254093; bh=+3L/UrHGl4azu4MImVkoZ8k0h0P1KlNAOyIZj1BowJw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=S6fTps3wt94rD7ifH2A+n+pqIqyZAAJaEfzqrgyvrq584ZkqODFDOCKTjjWrJIMMZ d2HlEtS6SfoPAid0E+caCpYj93asu5uOepavwFrlexsxeNyyWUhLcuFasZyXhcOMY9 iEgLsWA5HEk7iCo3kqaCm0mpRvGqKivsNHtULIfFLfKtcIljFjl+Gy3n9wWpRkkKa4 2hfpNRKK9SDn7W563kbmqpEi5BMBAO8/11ojjpFpRk86vfHnIzHUdYYObQjE5itROg 7dVLtUMYBGrHeCmgfIkwhNphdx7vEKfIBNAU1SiFxMSsR8WWWVUIr8YcdBVx0GTpbb c8xsaGsGc8MEg== Date: Tue, 28 Jul 2026 18:54:42 +0300 From: Mike Rapoport To: Peter Zijlstra Cc: "Lorenzo Stoakes (ARM)" , Dave Hansen , Dave Hansen , Andrew Morton , Andy Lutomirski , Borislav Petkov , David CARLIER , David Hildenbrand , Ingo Molnar , Jason Gunthorpe , Juergen Gross , Kevin Tian , Kiryl Shutsemau , "Liam R. Howlett" , Lu Baolu , "H. Peter Anvin" , Shakeel Butt , Suren Baghdasaryan , Thomas Gleixner , Toshi Kani , Vishal Moola , Vlastimil Babka , Will Deacon , iommu@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 1/5] x86/mm/pat: introcude cpa_lock() and cpa_unlock() Message-ID: References: <20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org> <20260728-cpa-fixes-v1-1-2ed2352300b3@kernel.org> <20260728142108.GW751831@noisy.programming.kicks-ass.net> <20260728143135.GG651302@noisy.programming.kicks-ass.net> <20260728145528.GI651302@noisy.programming.kicks-ass.net> <20260728150126.GC921102@noisy.programming.kicks-ass.net> <20260728153341.GY751831@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260728153341.GY751831@noisy.programming.kicks-ass.net> On Tue, Jul 28, 2026 at 05:33:41PM +0200, Peter Zijlstra wrote: > On Tue, Jul 28, 2026 at 04:20:10PM +0100, Lorenzo Stoakes (ARM) wrote: > > On Tue, Jul 28, 2026 at 05:01:26PM +0200, Peter Zijlstra wrote: > > > On Tue, Jul 28, 2026 at 04:55:28PM +0200, Peter Zijlstra wrote: > > > > On Tue, Jul 28, 2026 at 05:46:30PM +0300, Mike Rapoport wrote: > > > > > On Tue, Jul 28, 2026 at 04:31:35PM +0200, Peter Zijlstra wrote: > > > > > > On Tue, Jul 28, 2026 at 07:30:27AM -0700, Dave Hansen wrote: > > > > > > > On 7/28/26 07:21, Peter Zijlstra wrote: > > > > > > > > There was already a patch merged that removed the shole debug_pagealloc > > > > > > > > exception. Is that not better? > > > > > > > > > > > > > > As I'm scanning through email this morning, there's another issue that > > > > > > > popped up with that patch. It's causing hangs on boot. > > > > > > > > > > > > > > It's looking like debug pagealloc not taking the lock is actually > > > > > > > functional, not an optimization. Although, I hesitate to say > > > > > > > "functional" and would prefer to use much less nice words to describe it. > > > > > > > > > > > > Yeah, lets figure out why that is before we retain this wart ;-) > > > > > > > > > > As Lorenzo said: > > > > > > > > > > __kernel_map_pages() can be called from irq context: > > > > > > > > > > < GFP_ATOMIC context > > > > > > kfree() or whatever > > > > > -> ... > > > > > -> __free_pages_prepare() > > > > > -> debug_pagealloc_unmap_pages() > > > > > -> __kernel_map_pages() > > > > > -> __change_page_attr_set_clr() > > > > > -> cpa_lock > > > > > > > > > > > > > The TLBI hack in __kernel_map_pages() makes me wonder how any of this is > > > > correct to begin with. That comment isn't helping. > > > > > > > > > > Also, the 'atomic context' usage hereabout seems confused. In particular > > > the issue is with IRQ-disabled context, they are not the same thing. > > > > I mean yeah the IRQs being off is the issue with holding the lock over an IPI, a > > softirq allocating GFP_ATOMIC won't be a problem for that, but I think all the > > conclusions are still the same. > > preempt_disable() is an 'atomic context', but does not present the > problem. > > Anyway, yes not saying the conclusions are wrong, just that the wording > is confusing at heck. How would you like to word this? Is this one better? /* * When debug_pagealloc_enabled(), page attributes could be changed in a * context with IRQs disabled, so using spin_lock() with debug_pagealloc can * cause a deadlock. But since debug_pagealloc always uses 4k pages in the * direct map there are no races for splits and collapses and locking can * be just skipped altogether. */ -- Sincerely yours, Mike.