linux-kernel - Re: [RESEND PATCH v5] arm64: Enable vmalloc-huge with ptdump

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Message-ID: <aMrXBArFNLTdwWs3@willie-the-truck>
Date: Wed, 17 Sep 2025 16:43:00 +0100
From: Will Deacon <will@...nel.org>
To: Dev Jain <dev.jain@....com>
Cc: catalin.marinas@....com, anshuman.khandual@....com,
	quic_zhenhuah@...cinc.com, ryan.roberts@....com,
	kevin.brodsky@....com, yangyicong@...ilicon.com, joey.gouly@....com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	david@...hat.com, mark.rutland@....com, urezki@...il.com,
	jthoughton@...gle.com
Subject: Re: [RESEND PATCH v5] arm64: Enable vmalloc-huge with ptdump

On Tue, Sep 16, 2025 at 11:30:26AM +0100, Will Deacon wrote:
> I'm currently trying to put together a litmus test with James (cc'd) so
> maybe we can help you out with that part.

Here's what we came up with. There's not a good way to express the IPI
from kick_all_cpus_sync() but it turns out that the ISB from the TLB
invalidation is sufficient anyway. Does it make sense to you?


AArch64 ptdump
Variant=Ifetch
{
uint64_t pud=0xa110c;
uint64_t pmd;

0:X0=label:"P1:L0"; 0:X1=instr:"NOP"; 0:X2=lock; 0:X3=pud; 0:X4=pmd;
                    1:X1=0xdead;      1:X2=lock; 1:X3=pud; 1:X4=pmd;
}
 P0				| P1				;
 (* static_key_enable *)	| (* pud_free_pmd_page *)	;
 STR	W1, [X0]		| LDR	X9, [X3]		;
 DC	CVAU,X0			| STR	XZR, [X3]		;
 DSB	ISH			| DSB	ISH			;
 IC	IVAU,X0			| ISB				;
 DSB	ISH			|				;
 ISB				| (* static key *)		;
				| L0:				;
 (* mmap_lock *)		| B	out1			;
 Lwlock:			|				;
 MOV	W7, #1			| (* mmap_lock *)		;
 SWPA	W7, W8, [X2]		| Lrlock:			;
				| MOV	W7, #1			;
				| SWPA	W7, W8, [X2]		;
 (* walk pgtable *)		|				;
 LDR	X9, [X3]		| (* mmap_unlock *)		;
 CBZ	X9, out0		| STLR	WZR, [X2]		;
 EOR	X10, X9, X9		|				;
 LDR	X11, [X4, X10]		| out1:				;
				| EOR	X10, X9, X9		;
 out0:				| STR	X1, [X4, X10]		;

exists (0:X8=0 /\ 1:X8=0 /\	(* Lock acquisitions succeed *)
	0:X9=0xa110c /\		(* P0 sees the valid PUD ...*)
	0:X11=0xdead)		(* ... but the freed PMD *)


Will