linux-kernel - Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering tlb flush when folios get unmapped

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Date: Fri, 31 May 2024 23:09:46 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Byungchul Park <lkml.byungchul.park@...il.com>,
	Byungchul Park <byungchul@...com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel_team@...ynix.com,
	akpm@...ux-foundation.org, ying.huang@...el.com,
	vernhao@...cent.com, mgorman@...hsingularity.net, hughd@...gle.com,
	david@...hat.com, peterz@...radead.org, luto@...nel.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, rjgolo@...il.com
Subject: Re: [PATCH v11 09/12] mm: implement LUF(Lazy Unmap Flush) defering
 tlb flush when folios get unmapped

On Fri, May 31, 2024 at 02:46:23PM -0700, Dave Hansen wrote:
> On 5/31/24 11:04, Byungchul Park wrote:
> ...
> > I don't believe you do not agree with the concept itself.  Thing is
> > the current version is not good enough.  I will do my best by doing
> > what I can do.
> 
> More performance is good.  I agree with that.
> 
> But it has to be weighed against the risk and the complexity.  The more
> I look at this approach, the more I think this is not a good trade off.
> There's a lot of risk and a lot of complexity and we haven't seen the
> full complexity picture.  The gaps are being fixed by adding complexity
> in new subsystems (the VFS in this case).
> 
> There are going to be winners and losers, and this version for example
> makes file writes lose performance.
> 
> Just to be crystal clear: I disagree with the concept of leaving stale
> TLB entries in place in an attempt to gain performance.

FWIW, I agree with Dave.  This feels insanely dangerous and I don't
think you're paranoid enough about things that can go wrong.