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Date: Tue, 28 Jul 2026 17:46:30 +0300
From: Mike Rapoport <rppt@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	David CARLIER <devnexen@gmail.com>,
	David Hildenbrand <david@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	Juergen Gross <jgross@suse.com>, Kevin Tian <kevin.tian@intel.com>,
	Kiryl Shutsemau <kas@kernel.org>,
	"Liam R. Howlett" <liam@infradead.org>,
	Lorenzo Stoakes <ljs@kernel.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Suren Baghdasaryan <surenb@google.com>,
	Thomas Gleixner <tglx@kernel.org>, Toshi Kani <toshi.kani@hpe.com>,
	Vishal Moola <vishal.moola@gmail.com>,
	Vlastimil Babka <vbabka@kernel.org>, Will Deacon <will@kernel.org>,
	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: <amjAxhL-IeeJITEv@kernel.org>
References: <20260728-cpa-fixes-v1-0-2ed2352300b3@kernel.org>
 <20260728-cpa-fixes-v1-1-2ed2352300b3@kernel.org>
 <20260728142108.GW751831@noisy.programming.kicks-ass.net>
 <adfffb36-9943-4af7-a0ee-0e762e47fa11@intel.com>
 <20260728143135.GG651302@noisy.programming.kicks-ass.net>
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In-Reply-To: <20260728143135.GG651302@noisy.programming.kicks-ass.net>

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


-- 
Sincerely yours,
Mike.