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: Sat, 1 Jun 2024 11:20:08 +0900
From: Byungchul Park <lkml.byungchul.park@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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, 
	willy@...radead.org, 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

Dave Hansen <dave.hansen@...el.com> 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

All the complexity comes from the fact that I can't use a new space in
struct page - that can make the design even lockless.

I agree that keeping things simple is the best but I don't think all the
existing fields in struct page are the result of trying to make things
simple that you love.  Some of them are more complicated.

I'd like to find a better way together instead of yelling "it's unworthy
cuz it's too complicated and there's too little space in mm world to
accommodate new things".

However, for the issues already discussed, I will think about it more
before the next spin.

        Byungchul

> 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.
>