linux-kernel - Re: [PATCH v9 01/12] x86/mm: make MMU_GATHER_RCU_TABLE

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Message-ID: <CA+i-1C2QgSTq4gbGZqjyzGcuscsFZvymHvXnfJYuT-CNjPPuMQ@mail.gmail.com>
Date: Fri, 7 Feb 2025 15:28:31 +0100
From: Brendan Jackman <jackmanb@...gle.com>
To: Rik van Riel <riel@...riel.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, bp@...en8.de, 
	peterz@...radead.org, dave.hansen@...ux.intel.com, zhengqi.arch@...edance.com, 
	nadav.amit@...il.com, thomas.lendacky@....com, kernel-team@...a.com, 
	linux-mm@...ck.org, akpm@...ux-foundation.org, jannh@...gle.com, 
	mhklinux@...look.com, andrew.cooper3@...rix.com, 
	Manali Shukla <Manali.Shukla@....com>
Subject: Re: [PATCH v9 01/12] x86/mm: make MMU_GATHER_RCU_TABLE_FREE unconditional

Hi Rik,

On Thu, 6 Feb 2025 at 05:45, Rik van Riel <riel@...riel.com> wrote:
> This is done because some paravirt TLB flush implementations
> handle the TLB flush in the hypervisor, and will do the flush
> even when the target CPU has interrupts disabled.
>
> Always handle page table freeing with MMU_GATHER_RCU_TABLE_FREE.
> Using RCU synchronization between page table freeing and get_user_pages_fast()
> allows bare metal to also do TLB flushing while interrupts are disabled.
>
> Various places in the mm do still block IRQs or disable preemption
> as an implicit way to block RCU frees.
>
> That makes it safe to use INVLPGB on AMD CPUs.

It would be nice to update the CONFIG_MMU_GATHER_RCU_TABLE_FREE
comment in mm/mmu_gather.c to mention INVLPG alongside "Architectures
that do not have this (PPC)" - and while that's being updated it would
also be useful to note down the paravirt thing you explained above,
IMO it's pretty helpful to have more examples of the concrete usecases
for this logic.