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Date: Mon, 7 Jul 2025 14:49:54 +0300
From: Mike Rapoport <rppt@kernel.org>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Daniel Gomez <da.gomez@samsung.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Petr Pavlu <petr.pavlu@suse.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-modules@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH 1/8] execmem: drop unused execmem_update_copy()
Message-ID: <aGu0Yj08EZvpL5Xv@kernel.org>
References: <20250704134943.3524829-1-rppt@kernel.org>
 <20250704134943.3524829-2-rppt@kernel.org>
 <7e52f721-1d8e-4c50-af33-bee3f0d2ac6e@csgroup.eu>
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In-Reply-To: <7e52f721-1d8e-4c50-af33-bee3f0d2ac6e@csgroup.eu>

On Mon, Jul 07, 2025 at 12:10:43PM +0200, Christophe Leroy wrote:
> 
> Le 04/07/2025 à 15:49, Mike Rapoport a écrit :
> > From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> > 
> > The execmem_update_copy() that used text poking was required when memory
> > allocated from ROX cache was always read-only. Since now its permissions
> > can be switched to read-write there is no need in a function that updates
> > memory with text poking.
> 
> Erm. Looks like I missed the patch that introduced this change.
> 
> On some variant of powerpc, namely book3s/32, this is not feasible.

The only user of EXECMEM_ROX_CACHE for now is x86-64, we can always revisit
when powerpc book3s/32 would want to opt in to cache usage.

And it seems that [MODULES_VADDR, MODULES_END] is already mapped with
"large pages", isn't it?

> The granularity for setting the NX (non exec) bit is 256 Mbytes sections.
> So the area dedicated to execmem [MODULES_VADDR; MODULES_END[ always have
> the NX bit unset.
> 
> You can change any page within this area from ROX to RWX but you can't make
> it RW without X. If you want RW without X you must map it in the VMALLOC
> area, as VMALLOC area have NX bit always set.

So what will happen when one callse

	set_memory_nx()
	set_memory_rw()

in such areas?

> Christophe

-- 
Sincerely yours,
Mike.