linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

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Message-ID: <20230406150213.GQ386572@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2023 17:02:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Yair Podemsky <ypodemsk@...hat.com>, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
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        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
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        alougovs@...hat.com, jannh@...gle.com
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

On Thu, Apr 06, 2023 at 04:04:23PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 03:29:28PM +0200, Peter Zijlstra wrote:
> > On Thu, Apr 06, 2023 at 09:38:50AM -0300, Marcelo Tosatti wrote:
> > 
> > > > To actually hit this path you're doing something really dodgy.
> > > 
> > > Apparently khugepaged is using the same infrastructure:
> > > 
> > > $ grep tlb_remove_table khugepaged.c 
> > > 	tlb_remove_table_sync_one();
> > > 	tlb_remove_table_sync_one();
> > > 
> > > So just enabling khugepaged will hit that path.
> > 
> > Urgh, WTF..
> > 
> > Let me go read that stuff :/
> 
> At the very least the one on collapse_and_free_pmd() could easily become
> a call_rcu() based free.
> 
> I'm not sure I'm following what collapse_huge_page() does just yet.

DavidH, what do you thikn about reviving Jann's patches here:

  https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1

Those are far more invasive, but afaict they seem to do the right thing.