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Date: Sat, 11 Jul 2026 11:47:11 +0300
From: Mike Rapoport <rppt@kernel.org>
To: Dave Hansen <dave.hansen@intel.com>
Cc: "Denis V. Lunev" <den@virtuozzo.com>, "Denis V. Lunev" <den@openvz.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Juergen Gross <jgross@suse.com>, Kiryl Shutsemau <kas@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/mm/pat: take cpa_lock around large-page collapse
Message-ID: <alIDD0_ggoIovmTX@kernel.org>
References: <20260626163213.2284080-1-den@openvz.org>
 <d473612f-5bbc-4c7a-8587-363f1008f355@virtuozzo.com>
 <a33dce28-596f-4a4b-bd39-f3b2ad9f0d73@intel.com>
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In-Reply-To: <a33dce28-596f-4a4b-bd39-f3b2ad9f0d73@intel.com>

On Fri, Jul 10, 2026 at 10:15:54AM -0700, Dave Hansen wrote:
> cpa_collapse_large_pages() seems to be a never ending source of bugs.

Yeah, this one, the race with ptdump and a year old issue on Xen PV.
There might be more of course but I wouldn't call it endless.

> I'm tempted to say we should just axe it, call it a failed experiment,
> and move on with life.
> 
> Is it worth keeping?

This what lets keeping executable mappings in 2M granularity. Modules,
ftrace and kprobes are already using it and BPF is on the way: 

https://lore.kernel.org/bpf/20260626-execmem-x86-rox-bpf-v0-v1-0-45a0b0ed4fe9@kernel.org/

-- 
Sincerely yours,
Mike.