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

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Message-ID: <20250103121843.GDZ3fVo0r0JRsGImBS@fat_crate.local>
Date: Fri, 3 Jan 2025 13:18:43 +0100
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	dave.hansen@...ux.intel.com, luto@...nel.org, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, akpm@...ux-foundation.org,
	nadav.amit@...il.com, zhengqi.arch@...edance.com,
	linux-mm@...ck.org
Subject: Re: [PATCH 01/12] x86/mm: make MMU_GATHER_RCU_TABLE_FREE
 unconditional

On Thu, Jan 02, 2025 at 08:56:09PM +0100, Peter Zijlstra wrote:
> Well, I've already answered why we need this in the previous thread but
> it wasn't preserved :-(

... and this needs to be part of the commit message. And there's a similar
comment over tlb_remove_table_smp_sync() in mm/mmu_gather.c which pretty much
explains the same thing.

> Currently GUP-fast serializes against table-free by disabling
> interrupts, which in turn holds of the TLBI-IPIs.
> 
> Since you're going to be doing broadcast TLBI -- without IPIs, this no
> longer works and we need other means of serializing GUP-fast vs
> table-free.
> 
> MMU_GATHER_RCU_TABLE_FREE is that means.
> 
> So where previously paravirt implementations of tlb_flush_multi might
> require this (because of virt optimizations that avoided the TLBI-IPI),
> this broadcast invalidate now very much requires this for native.

Right, so this begs the question: we probably should do this dynamically only
on TLBI systems - not on everything native - due to the overhead of this
batching - I'm looking at tlb_remove_table().

Or should we make this unconditional on all native because we don't care about
the overhead and would like to have simpler code. I mean, disabling IRQs vs
batching and allocating memory...?

Meh.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette