linux-kernel - Re: [PATCH] x86/mm/cpa: get rid of the cpa lock

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Message-ID: <Y7f9ZuPcIMk37KnN@gmail.com>
Date:   Fri, 6 Jan 2023 11:52:22 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Jacky Li <jackyli@...gle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Marc Orr <marcorr@...gle.com>,
        Alper Gun <alpergun@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] x86/mm/cpa: get rid of the cpa lock


* Jacky Li <jackyli@...gle.com> wrote:

> It’s true that with such old code, the cpa_lock might protect more
> race conditions than those that it was introduced to protect in 2008,
> or some old hardware may depend on the cpa_lock for undocumented
> behavior. So removing the lock directly might not be a good idea, but
> it probably should not mean that we need to keep the inefficient code
> forever. I would appreciate any suggestion to navigate this lock
> removal from the folks on the to and cc list.

> -/*
> - * Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings)
> - * using cpa_lock. So that we don't allow any other cpu, with stale large tlb
> - * entries change the page attribute in parallel to some other cpu
> - * splitting a large page entry along with changing the attribute.
> - */
> -static DEFINE_SPINLOCK(cpa_lock);

Yeah, so I'm *really* tempted to just remove cpa_lock if there's no in-code 
documented uses of it - your patch provides *exhaustive* background.

The thing is, even in the worst-case if it breaks anything, it will get 
investigated, documented better and maybe reverted - which would *still* be 
an improvement over today, because we turn undocumented code into 
documented code.

We cannot indefinitely keep a global lock just because we fear it might 
have some undocumented dependencies...

But no strong feelings either way - I've added a few more Cc:s to discuss 
this more widely.

Unless there's objections I'd be inclined to give this patch a try, and 
keep an eye open for regressions, it's not difficult to revert either.

Thanks,

	Ingo